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Week 16 · Final study guide

Final Exam — Study Guide

Principles of Microeconomics Free sample edition

Coverage — what the final covers

The final is cumulative over Weeks 1–15, and it checks all eight course objectives. It opens in Week 16, the finals week, and closes at the due time your instructor sets. There is no time limit, and you get one attempt. The questions appear one at a time, and you cannot go back once you move on.

Format: 40 questions, and every question counts the same. Thirty-one ask you to pick the one option that fits, five ask you to select every option that fits, and four ask you to judge a statement true or false. Every graph a question needs is described in words, and every schedule, cost table, demand line, and payoff matrix it uses is written out in the question. Several questions need a short calculation: a subtraction down a table, a product, one half of a base times a height, or two straight lines set equal. Scratch paper is enough for all of them.

What is in scope, with the number of final questions on each course objective:

Final coverage by course objective: the weeks that teach it, its main skills, and its number of questions out of 40
Course objective Weeks that teach it Main skills Questions
1. Economic modeling, positive and normative 1 and 2, carried into 9 and 15 Opportunity cost, the PPF, comparative advantage, and sorting a claim as positive or normative 3
2. Demand, supply, and equilibrium 3 and 4, carried into 13 Shift or movement along a curve; solving for the equilibrium; naming the curve, the direction, and the new price and quantity 2
3. Elasticity 5, carried into 7 The midpoint formula, the revenue test, and responsiveness as the reason one side bears more of a tax 1
4. Surplus and intervention 6 and 7, carried into 11 and 14 Consumer and producer surplus, deadweight loss triangles, taxes, and subsidies 3
5. Production and costs 9, carried into 10 Total, marginal, and average product; the cost table; accounting profit against economic profit 6
6. Perfect competition and monopoly 10 and 11, carried into 12 The output rule for each market structure, the shutdown rule, entry and exit, monopoly pricing 12
7. Differentiated sellers, oligopoly, and labor 12 and 13 Differentiated sellers, payoff matrices, dominant strategies, Nash equilibrium, and the hiring rule 6
8. Market failure, behavior, and inequality 14 and 15 Externalities and their fixes, the four kinds of goods, hidden information, behavioral patterns, the Lorenz curve 7

Where the weight sits. The syllabus says the final gives the most weight to Objectives 5 to 8 while still drawing on every first-half objective, and this form leans that way strongly. Thirty-one questions check Objectives 5 to 8, and nine check Objectives 1 to 4. Thirty-nine of the 40 questions come from the material of Weeks 9–15, and one comes from Week 7. Week 10 carries the most, 7 questions, so put the most review time on the price taker's output rule, profit at a given price, the shutdown rule, and entry and exit. Weeks 11 and 14 come next with 6 each, then Weeks 9, 12, 13, and 15 with 5 each. By objective, Objective 6, perfect competition and monopoly, is by far the heaviest at 12 questions, followed by Objective 8 with 7. By difficulty, 11 questions are easy, 22 are medium, and 7 are hard.

The first half still matters. Nine questions check Objectives 1 to 4, and eight of them sit inside a later week's material, the way those weeks called the early tools back. Week 9 priced an owner's own time and money as opportunity cost, the Week 1 idea. Week 11 measured monopoly's lost surplus with Week 6's triangles, and Week 14 did the same for an externality and brought back Week 7's per-unit tax as the fix. Week 13 ran the demand and supply model of Weeks 3 and 4 on a labor market. Week 15 sorted the term's debates into positive and normative claims, the Week 1 line. The one remaining question comes from Week 7 itself, on who bears a tax and why responsiveness decides it. No question comes from Week 8's own material, but its habit of thinking at the margin runs through every firm week. So Weeks 1–8 get a short callback section below, and Weeks 9–15 get the full review.

Topic-by-topic review

Each topic gives the key idea in plain words, the course's own definitions and pinned examples, and the mistakes students make most often, each with its cure. The deeper review for every topic is that week's chapter, lecture page, slide deck, and readings. Every dataset named below was constructed for this course; it is illustrative, not real data.

Weeks 1–8 in brief — the tools the later weeks reuse

  • Opportunity cost (Week 1): the value of the single best alternative you give up, counting time and forgone earnings as well as any money paid. The pinned case: a 60-dollar concert ticket plus 4 hours away from a 15-dollar-an-hour shift costs 60 + 4 × 15 = 120 dollars, not 60. Week 9 reuses this idea as the implicit cost of an owner's own time and savings.
  • The PPF (Week 1): a point on the frontier is efficient, a point inside is attainable but wasteful, and a point outside is out of reach for now. The slope is the opportunity cost, and a bowed-out shape means that cost rises.
  • Comparative advantage (Week 2): the producer with the lower opportunity cost of a good has the comparative advantage in it, even if the other producer is faster at everything. Terms of trade help both sides only inside the range between their two opportunity costs.
  • Positive and normative (Week 1): a positive statement is a claim about what is, or what would happen, that evidence can check, at least in principle. A normative statement is a claim about what ought to be; it rests on values, and evidence alone cannot settle it. Positive means testable, not good news, and normative means value-laden, not bad news.
  • Shift or movement (Weeks 3–4): a change in the good's own price moves buyers or sellers along an unchanged curve (a change in quantity demanded or quantity supplied). A change in anything else shifts the whole curve (a change in demand or supply). After a shift, name the curve, the direction, and the new price and quantity. On Dataset A, the running market, Qd = 90 − 10P and Qs = 10P − 30 cross at 6 dollars and 30 units a day.
  • Elasticity (Week 5): the price elasticity of demand is the percentage change in quantity demanded divided by the percentage change in price, each change taken against the average of its two values (the midpoint formula). Demand is elastic when the magnitude is above 1 and inelastic when it is below 1. Elasticity is not the slope.
  • Surplus (Week 6): consumer surplus is the area below demand and above the price; producer surplus is the area above supply and below the price. On Dataset A each is 1⁄2 × 30 × 3 = 45 dollars a day, for a total of 90. The competitive equilibrium makes total surplus as large as it can be in the model, and that is a positive property, not a verdict on fairness.
  • Controls and taxes (Week 7): a ceiling binds only below the equilibrium price and leaves a shortage; a floor binds only above it and leaves a surplus. A per-unit tax drives a wedge between the price buyers pay and the price sellers keep. On Dataset A a 2-dollar tax has buyers paying 7, sellers keeping 5, and 20 units trading; it raises 40 dollars and destroys 1⁄2 × 2 × 10 = 10 dollars of surplus. Economic incidence is how the burden is really shared, and the side less responsive to price bears more, whoever writes the check. In Dataset J buyers bear 3 of a 4-dollar tax because their quantity responds less.
  • Thinking at the margin (Week 8): do one more unit while its marginal benefit is at least its marginal cost, and stop at the first unit whose extra cost is larger. A sunk cost is spending already made that cannot be recovered, so it is the same under every option and never enters the choice. In Dataset U the café stays open 3 extra hours for a net gain of 60 dollars, and the rent it already paid plays no part.

Week 9 — Production and Costs

  • The distinctions: short run vs. long run; marginal vs. average product; fixed vs. variable cost; marginal vs. average cost; accounting vs. economic profit.
  • Key idea: open the firm and read its numbers. In the short run at least one input is fixed, such as one kitchen or one room, so output grows only by adding more of the other inputs. In the long run every input can change.
  • Product: total product is all the output a given number of workers makes. Marginal product is the extra output from one more worker, with everything else fixed. Average product is output per worker, total output divided by the number of workers. In Dataset K the bakery makes 0, 10, 25, 45, 60, 70, and 75 loaves with 0 to 6 workers, so marginal product is 10, 15, 20, 15, 10, and 5.
  • The law of diminishing marginal returns: as more of a variable input is added to a fixed one, its marginal product eventually falls. It begins after some point, here with the fourth worker, not the first, and it says nothing about anyone being lazy. Total output can keep rising while marginal product falls.
  • Costs: fixed cost does not change with output in the short run, and the firm pays it even at zero output. Variable cost rises and falls with output and is zero at zero output. TC = FC + VC. Average fixed cost is FC ⁄ Q and falls with every unit; average variable cost is VC ⁄ Q; average total cost is TC ⁄ Q, which also equals AFC + AVC. Marginal cost is the change in total cost from one more unit, and because fixed cost never changes, it equals the change in variable cost.
  • Marginal pulls the average: a marginal value below an average pulls the average down, and one above it pulls the average up. That is why marginal cost crosses average variable and average total cost at their lowest points. In Dataset L's chair workshop, with a fixed cost of 60, marginal cost is 30, 20, 16, 22, 32, and 48; average variable cost is lowest at 22 (3 and 4 chairs) and average total cost is lowest at 36 (5 chairs).
  • Profit: an explicit cost is money paid out, such as rent, wages, or materials. An implicit cost is the opportunity cost of what the owner supplies without a bill: the pay the owner gives up and the return the owner's savings could have earned. Accounting profit is revenue minus explicit costs; economic profit is revenue minus both kinds. The pinned phone-repair stand shows an accounting profit of 30,000 dollars and an economic loss of 7,000 once its owner's 37,000 dollars of implicit costs count.
  • Predictable mistakes → cures: marginal cost confused with average total cost → ask "what does one more unit add?" for marginal and "what is the cost per unit so far?" for average. Diminishing returns said to start with the first worker → find the first worker whose extra output is smaller than the one before. Fixed cost put into the cost of one more unit → fixed cost does not change, so it cannot be part of a change. Accounting profit called economic profit → subtract the owner's forgone pay and interest before you call anything a profit.

Week 10 — Perfect Competition

  • The distinctions: price taker vs. price setter; P = MC vs. MR = MC; the shutdown price vs. the break-even price; accounting vs. economic profit at zero.
  • Key idea: a market in perfect competition has many buyers and sellers, an identical product, buyers and sellers who know the going price, and free entry and exit. No single seller can move the price, so each firm is a price taker: it can sell all it wants at the market price and nothing at a higher one. Its demand curve is a flat line at the market price.
  • The output rule: every profit-maximizing firm produces where MR = MC, the output where marginal revenue and marginal cost are equal. For a price taker each unit adds the market price to revenue, so MR = P and the rule reads P = MC, on the rising part of the marginal-cost curve. In Dataset M the chair workshop makes 5 chairs at 40 dollars, since the 5th chair adds 32 dollars of cost and the 6th adds 48.
  • Profit at a price: profit is total revenue minus total cost, which also equals (P − ATC) × Q, a rectangle on the graph. At 40 dollars the workshop earns 200 − 180 = 20 dollars, or (40 − 36) × 5 = 20.
  • Two thresholds: the break-even price is the lowest point of average total cost, 36 dollars for the workshop, where economic profit is zero. The shutdown price is the lowest point of AVC, 22 dollars here. Between the two, the firm loses money but stays open: at 30 dollars it loses 28 by operating and would lose its whole 60-dollar fixed cost by closing. Below 22, it closes and loses only the fixed cost.
  • The long run: economic profit draws new firms in, market supply shifts right, and the price falls. Losses push firms out, market supply shifts left, and the price rises. Entry and exit stop when the price reaches the bottom of the ATC curve and economic profit is zero. Zero economic profit does not mean the owner earns nothing; it means the owner's time and money do as well here as in their best other use, and the books still show an accounting profit.
  • Three costs, three jobs: marginal cost decides how many units; average variable cost decides whether to open; average total cost decides whether there is a profit.
  • Predictable mistakes → cures: shutting down at any loss → compare the loss from operating with the fixed cost you lose by closing. Setting output where price meets average total cost → output comes from marginal cost; average total cost only tells you whether you profit. A competitive firm said to choose its price → it takes the price and chooses only the quantity.

Week 11 — Monopoly

  • The distinctions: P = MC vs. MR = MC; the price read from demand vs. read from the MR line; profit vs. deadweight loss; a positive claim vs. a normative one in the antitrust debate.
  • Key idea: a monopoly is the only seller of a product with no close substitutes, protected by barriers to entry: control of a key resource, legal protection such as patents and licenses, or economies of scale so large that one firm serves the whole market most cheaply (a natural monopoly).
  • Marginal revenue below price: to sell more, a monopolist must lower its price on every unit, not only the new ones, so marginal revenue sits below the price at every quantity after the first. In Dataset N the town's only ferry faces P = 20 − Q (Q counts tickets per day, in hundreds); its marginal revenue runs 19, 17, 15, and so on down by 2, the straight line MR = 20 − 2Q.
  • The rule and the price: the ferry sells where MR = MC: 20 − 2Q = 4 gives 8 hundred tickets. It then reads its price from the demand curve, 20 − 8 = 12 dollars, not the 4 dollars where the two lines meet. Profit is (12 − 4) × 8 = 64, that is, 6,400 dollars a day. Demand limits the monopolist: at 20 dollars it would sell nothing.
  • The deadweight loss: competitive boats would run at P = MC, selling 1,600 tickets a day at a 4-dollar fare, where riders' surplus is 1⁄2 × 16 × 16 = 128. Under the monopoly it falls to 32. The ferry's 64 is a transfer from riders, and the rest, 1⁄2 × 8 × 8 = 32, counts as deadweight loss on trips that never happen. The check: 32 + 64 + 32 = 128.
  • The Lerner index: the markup over marginal cost as a share of the price, (P − MC) ⁄ P, which Lerner set out in 1934. For the ferry it is 8 ⁄ 12, two-thirds: two-thirds of each fare is markup over the cost of one more rider. A price taker's index is zero.
  • Price discrimination: selling the same good at different prices to different buyers for reasons unrelated to cost. It needs three things: market power, a way to tell buyers who will pay more from buyers who will pay less, and a way to stop resale.
  • Antitrust as a named debate: the Sherman Act (1890) and the Federal Trade Commission Act and Clayton Act (both 1914) are the core federal antitrust laws, and Section 7 of the Clayton Act prohibits mergers whose effect "may be substantially to lessen competition, or to tend to create a monopoly." One side argues for vigorous enforcement because market power shrinks output and raises prices. The other argues for restraint because economies of scale and the reward for invention can serve buyers. How hard to enforce is a normative choice, and the course gives no verdict.
  • Predictable mistakes → cures: P = MC given to a monopolist → P = MC is the price taker's reading of MR = MC, and only there does MR equal the price. The price read off the MR line → go up from the chosen quantity to the demand curve. "A monopolist charges whatever it wants" → buyers still respond to price, so demand caps it. Profit counted as deadweight loss → profit moves to the seller; the deadweight loss goes to no one.

Week 12 — Differentiated Sellers and Oligopoly

  • The distinctions: a monopolistically competitive market vs. an oligopoly; the cooperative outcome vs. the Nash equilibrium; a dominant strategy vs. the biggest number in the table.
  • Monopolistically competitive markets: many sellers, products that differ a little by location, style, speed, quality, or service, and free entry. Each seller faces its own downward-sloping demand, produces where MR = MC, and reads its price from that demand, so its price sits above marginal cost. Edward Chamberlin and Joan Robinson each set out this framework in 1933.
  • The long-run zero-profit result: profit draws in new sellers with close substitutes, each one takes some buyers, and every existing seller's demand shifts left until P = ATC and economic profit is zero. The price still sits above marginal cost; buyers get variety in return.
  • Oligopoly: a few firms whose choices are interdependent, usually behind barriers to entry. Collusion is an agreement among rivals to raise prices or cut output; a formal one is a cartel. Collusion is unstable, because each member gains by secretly cutting its price or selling more, and price-fixing agreements are illegal under U.S. antitrust law.
  • Game theory: a payoff matrix lists each player's payoff for every pair of choices, written (row player, column player). A dominant strategy pays more than any other choice whatever the other player does; find it by comparing the player's own payoffs within each of the other player's choices. A Nash equilibrium is a set of choices in which no player can do better by changing only its own choice, the idea John Nash published in 1950.
  • Dataset O: the two coffee shops earn (10, 10) if both price high, (4, 14) or (14, 4) if they split, and (6, 6) if both price low, all in thousand-dollar units per month. Low is each shop's dominant strategy, so the game's Nash equilibrium is (low, low), at 6 each, though both would earn 10 at (high, high). That gap is the prisoner's dilemma, a game Merrill Flood reported in 1958: each player follows its dominant strategy, and both end up worse off than if both had cooperated.
  • The four structures compared: perfect competition (many firms, identical product, free entry, P = MC, zero long-run profit); monopolistically competitive markets (many firms, differentiated products, free entry, price above marginal cost, zero long-run profit); oligopoly (a few interdependent firms, barriers, profit that can last); monopoly (one firm, no close substitutes, blocked entry, profit that can last). All four produce where MR = MC; only the price taker's rule reads P = MC.
  • Predictable mistakes → cures: the cooperative outcome called the Nash equilibrium → test each cell by asking whether either player would switch alone. A dominant strategy read as the biggest number → compare within one column at a time. Long-run profit expected for a differentiated seller → entry is free, so profit is competed away. An oligopoly confused with a monopolistically competitive market → count the sellers and ask whether each must watch the others.

Week 13 — Factor and Labor Markets

  • The distinctions: marginal product vs. its value in dollars; a movement along labor demand vs. a shift of it; the rising vs. the falling part of the schedule; a positive vs. a normative claim about wages.
  • Key idea: the firm turns into a buyer. Labor, land, and capital are the factors of production, and labor demand is a derived demand: a bakery wants bakers because buyers want bread.
  • Dollars, not loaves: the value of labor's marginal product is the output price times the marginal product, VMPL = P × MP. At 4 dollars a loaf, Dataset K's six workers are worth 40, 60, 80, 60, 40, and 20 dollars a day.
  • The hiring rule: a firm that takes the wage as given hires while VMPL is at least the wage, on the part of the schedule where marginal product is falling. At a 50-dollar wage the bakery hires 4 (the fourth adds 60, the fifth only 40). The first worker's 40 dollars sits on the rising part, so the bakery does not stop at zero.
  • Movement or shift: a change in the wage moves the firm along its labor demand curve: at a 30-dollar wage the bakery hires 5. A change in the output price or in productivity shifts labor demand: if bread rises to 6 dollars a loaf, the values become 60, 90, 120, 90, 60, and 30, and the bakery hires 5 at the same 50-dollar wage.
  • The labor market: labor supply slopes up and shifts when the number of workers, pay in other jobs, tastes for work, or training changes. Where labor demand meets labor supply you find the market wage and the number employed, and you name the curve, the direction, and the new wage and the number employed, the same habit as in Weeks 3 and 4.
  • Why wages differ: human capital is the knowledge and skill a worker builds through education, training, and experience. A compensating differential is extra pay for a job's risk, unpleasant tasks, or odd hours. Union bargaining and discrimination are other forces economists measure and debate.
  • The minimum wage as a named debate: the model predicts fewer jobs at a binding floor. Studies disagree about the size of the effect: Card and Krueger (1994), and their reply in 2000, found no fall in New Jersey fast-food jobs after the state raised its minimum, while a study by Neumark and Wascher, also from 2000, found one. Unions and gig work are debated the same way. The course grades the model and the labels, never a verdict.
  • Predictable mistakes → cures: hiring until marginal product equals the wage → turn the extra output into dollars first. A wage change drawn as a shift of labor demand → the wage is labor's own price, so it moves the firm along the curve. Applying the rule on the rising part → start where marginal product has begun to fall.

Week 14 — Externalities, Public Goods, and Market Failure

  • The distinctions: private vs. social cost; too much vs. too little; a per-unit tax vs. a total harm; who pays vs. what is produced; a public good vs. a good the government provides.
  • Key idea: an externality is a cost or benefit that falls on people outside a transaction, who neither buy nor sell the good. A negative one, such as smoke, is a cost; a positive one, such as a vaccination that protects neighbors, is a benefit.
  • Marginal social cost: MSC = MPC + MEC, the seller's own marginal cost plus its marginal external cost (MEC), the harm one more unit does to bystanders. The efficient quantity, where marginal social benefit and MSC cross, is the target. In Dataset P each unit of the running market's good does 2 dollars of harm, so MSC = 5 + Q ⁄ 10; it meets demand, 9 − Q ⁄ 10, at 20 units and 7 dollars, while the market ignores the harm and trades 30 at 6. A negative externality makes the market produce too much.
  • The deadweight loss: on the 10 extra units the cost to society exceeds what buyers value them at by a gap that grows from 0 to 2 dollars, so the loss is the triangle 1⁄2 × 2 × 10 = 10 dollars a day.
  • The corrective tax: a per-unit tax equal to MEC at the efficient level of output, an idea credited to Pigou (1920). Week 7's same 2-dollar tax restores 20 units here: in Week 7, with no externality, it caused a deadweight loss of 10; here it removes one of 10. With 2 dollars of external benefit instead, the efficient output becomes 40 and a 2-dollar subsidy reaches it, because a positive externality makes the market produce too little.
  • Cap-and-trade: cap the total pollution and let firms trade permits, so the cutting moves to whoever can cut most cheaply. In the EPA's Acid Rain Program each allowance permits one ton of sulfur dioxide.
  • The Coase theorem (Coase, 1960): when property rights are clear and bargaining costs are low, private bargaining reaches the efficient outcome whichever side holds the right; the right decides who pays whom, not what is produced. In Dataset Q a 200-dollar filter stops 300 dollars of damage to the laundry, so it goes in either way.
  • Four kinds of goods: a rival good is one where one person's use leaves less for others; an excludable good is one that non-payers can be kept from using. Private goods are both, public goods are neither, common resources are rival but not excludable, and club goods are excludable but not rival. Samuelson (1954) set out the theory of public goods. The free-rider problem is the pull to enjoy a good no one can be kept from without paying for it: Dataset Q's fireworks show is worth 1,000 dollars to 100 residents and costs 600, yet donations can fall short.
  • The commons: Hardin (1968) named the tragedy of the commons, the overuse of a resource that is rival but open to all. Ostrom (1990) documented communities that manage shared resources well by their own rules.
  • Carbon pricing as a named debate: the positive claims are testable (a price on emissions raises the cost of goods that emit; firms that can cut cheaply cut first). The normative questions rest on values: how much weight future people get, who should bear the cost, and whether a tax, a cap, or rules on technology is the better tool. The course gives no verdict.
  • Predictable mistakes → cures: a negative externality said to make too little → the seller ignores part of the cost, so it makes too many. The corrective tax set to the total harm → set it per unit, equal to the harm of one more unit. The Coase theorem read as "the right changes what is produced" → it changes who pays. Any government service called a public good → ask the two questions, rival and excludable, whoever provides it.

Week 15 — Asymmetric Information, Behavioral Economics, and Inequality

  • The distinctions: adverse selection vs. moral hazard; signaling vs. screening; a predictable pattern vs. a mistake; a Gini of 0 vs. a Gini of 1; positive vs. normative questions about inequality.
  • Asymmetric information: one side of a deal knows something the other side does not. Sort it by timing. Adverse selection comes from hidden characteristics before the deal; moral hazard comes from hidden actions after it.
  • The lemons market (Akerlof, 1970): in Dataset R good cars are worth 10,000 dollars to buyers and 9,000 to their owners, and lemons 4,000 and 3,000, half of each. An uninformed buyer offers the average, 7,000. Good-car owners refuse, only lemons are offered, and the price falls toward a lemon's value. Each good car that never trades is a lost gain of 1,000 dollars. The lemons result does not say all used cars are bad; it says good ones can be driven out when buyers cannot tell them apart.
  • Responses: a signal is an action by the informed side that would cost too much to fake, such as a warranty. Screening is an action by the uninformed side to sort the other side, such as an independent inspection. A deductible, the part of each loss the insured person pays first, is a common response to moral hazard.
  • Behavioral economics studies the predictable ways real people depart from the careful chooser in the models. Bounded rationality: limited time, attention, and information push people toward rules of thumb. Loss aversion: a loss weighs more than a gain of the same size, a central element of the prospect theory Kahneman and Tversky published in 1979. Present bias: costs and benefits that arrive now get far more weight than the same ones later. Anchoring: a first number pulls later judgments toward it. A default is the option that takes effect when a person makes no active choice; Madrian and Shea (2001) found that automatic enrollment raised retirement-plan participation. Whether anyone should set a default, and which one, is a normative question.
  • The Lorenz curve and the Gini coefficient: rank households from poorest to richest into five equal groups, the quintiles, and plot the cumulative share of income against the cumulative share of households. The straight diagonal is the line of perfect equality, and the farther the curve sags below it, the more unequal the shares. The Gini coefficient summarizes that sag; the Census Bureau's scale runs from 0 for perfect equality up to 1 when one group gets all the income. In Dataset S the shares are 5, 10, 15, 25, and 45 percent, the cumulative shares 5, 15, 30, 55, and 100, the area under the curve 0.31, and the Gini 1 − 2 × 0.31 = 0.38.
  • Inequality as a named debate: the positive questions are testable: how unequal incomes are, what explains it, and how taxes and transfers change choices to work and save. The normative questions rest on values: how much equality a society should aim for, and whether equal opportunity or equal outcomes matter more. One side stresses that redistribution can ease hardship and widen opportunity; the other stresses incentives and a fair process. The course gives no verdict.
  • The term's synthesis: markets allocate well when buyers and sellers face the full costs and benefits of their choices and share the same information (Week 6's 90 dollars of surplus). They fail with market power (the ferry's 32), externalities (the smoke's 10), public goods, and hidden information. For any policy, do the positive analysis first, then state the normative judgment and label it as one.
  • Predictable mistakes → cures: adverse selection and moral hazard swapped → ask whether the hidden thing existed before the deal or happened after it. A Gini of 0 read as perfect inequality → 0 means every fifth gets a fifth. A bias read as "people are stupid" → it is a pattern predictable enough to model.

Representative practice

These are fresh variants, not the final's questions. Work each worked example yourself first, then compare your steps. Every business, market, and number below is illustrative, invented for this guide and attributed to no real person or company, and every number was computed and checked in advance.

Worked example — a lunch-box kitchen adds workers (Week 9)

An illustrative community-center kitchen packs lunch boxes with one fixed set of counters. With 0 to 6 workers it packs 0, 14, 32, 54, 70, 80, and 84 boxes a shift.

  1. Marginal product. Subtract each total from the next: 14 − 0 = 14, 32 − 14 = 18, 54 − 32 = 22, 70 − 54 = 16, 80 − 70 = 10, and 84 − 80 = 4 boxes.
  2. Average product. Divide each total by the workers: 14 ⁄ 1 = 14, 32 ⁄ 2 = 16, 54 ⁄ 3 = 18, 70 ⁄ 4 = 17.5, 80 ⁄ 5 = 16, and 84 ⁄ 6 = 14 boxes per worker.
  3. Where diminishing returns begin. Marginal product rises from 14 to 18 to 22, then falls to 16 with the fourth worker, so diminishing marginal returns begin with the fourth worker.
  4. Marginal pulls the average. The fourth worker's 16 is below the average of 18 with three workers, so the average falls to 17.5.
  5. Say it in words. The fourth worker still adds 16 boxes, so total output keeps rising, but each new worker adds less because the counters stay the same size.

Why it matters: the falling marginal product is what makes marginal cost rise in the next example.

Worked example — a banner shop's cost table (Week 9)

An illustrative banner-printing shop pays a fixed cost of 48 dollars a day. Its variable cost is 0, 24, 40, 51, 64, 82, and 120 dollars for 0 to 6 banners a day.

  1. Total cost. TC = FC + VC: 48, 72, 88, 99, 112, 130, and 168 dollars.
  2. Marginal cost. The change in total cost: 24, 16, 11, 13, 18, and 38 dollars. The same six numbers come from variable cost alone, since the 48 dollars never changes.
  3. The averages. Average fixed cost is 48, 24, 16, 12, 9.6, and 8 dollars. Average variable cost is 24, 20, 17, 16, 16.4, and 20 dollars, lowest at 16 with 4 banners. ATC runs 72, 44, 33, 28, 26, and 28 dollars, lowest at 26 with 5 banners.
  4. Marginal pulls the average. The 5th banner's marginal cost of 18 is below the average total cost of 28 at 4 banners, so the average falls to 26. The 6th banner's 38 is above 26, so the average rises to 28.
  5. Say it in words. The 5th banner adds 18 dollars while the shop's cost per banner is 26: two different numbers that answer two different questions.

Why it matters: you will need the 16-dollar low point of AVC and the 26-dollar low point of ATC in the next example.

Worked example — the banner shop as a price taker (Weeks 10 and 8)

The banner shop sells in a market with many shops and identical banners, so it takes the price as given.

  1. At 30 dollars a banner. Marginal cost is 24, 16, 11, 13, 18, and 38. On the rising part, the 5th banner's 18 is no more than 30 and the 6th banner's 38 is more, so the shop makes 5. Revenue is 30 × 5 = 150 dollars and total cost is 130, a profit of 20, the same as (30 − 26) × 5 = 20.
  2. At 20 dollars a banner. The 5th banner's 18 is still no more than 20, so it makes 5. Revenue is 100 and total cost is 130, a loss of 30. Closing would lose the whole 48-dollar fixed cost, and revenue of 100 covers the 82 dollars of variable cost with 18 to spare, so the shop stays open and loses 30 instead of 48.
  3. At 14 dollars a banner. The price is below the lowest average variable cost of 16, so the shop shuts down and loses its fixed cost of 48. Making 4 banners would bring in 56 against a total cost of 112, a loss of 56, which is worse.
  4. The long run. At 30 dollars the shops earn economic profit, so new shops enter, market supply shifts right, and the price falls toward the 26-dollar low point of average total cost, where economic profit is zero.
  5. Say it in words. Marginal cost picked the quantity at every price; average variable cost decided whether to open; average total cost decided whether there was a profit.

Why it matters: this is the Week 8 margin habit run one banner at a time, with the fixed cost left out of every "one more" decision.

Worked example — a home photography business (Weeks 9 and 1)

An illustrative photographer brings in 64,000 dollars a year and pays 22,000 in explicit costs for equipment, travel, and insurance. She gave up a salaried job at 40,000 a year to do this work. She also took 25,000 out of a savings account paying 4 percent interest to buy her cameras, which gives up 0.04 × 25,000 = 1,000 dollars a year.

  1. Accounting profit. 64,000 − 22,000 = 42,000 dollars a year.
  2. Implicit costs. 40,000 + 1,000 = 41,000 dollars a year, the opportunity cost of her own time and savings.
  3. Economic profit. 64,000 − 22,000 − 41,000 = 1,000 dollars a year.
  4. Say it in words. The business earns 1,000 dollars a year more than her time and savings would earn in their next-best use, which is much less than the 42,000 the books show.

Why it matters: the Week 1 idea of opportunity cost is what turns accounting profit into economic profit.

Worked example — the only laundromat in a remote town (Weeks 11 and 6)

An illustrative laundromat is the only one for miles. Its demand is P = 19 − Q, with P in dollars a load and Q in hundreds of loads a week. Each extra load costs it 5 dollars in water and power, the same for every load, and it has no fixed cost.

  1. Marginal revenue. Total revenue for 1 to 9 hundred loads is 18, 34, 48, 60, 70, 78, 84, 88, and 90 hundred dollars, so marginal revenue is 18, 16, 14, 12, 10, 8, 6, 4, and 2: the line MR = 19 − 2Q.
  2. The output rule. The 7th hundred loads adds 6 to revenue and 5 to cost, so the laundromat sells it; the 8th adds only 4. On the line, 19 − 2Q = 5 gives Q = 7 hundred loads.
  3. The price from demand. 19 − 7 = 12 dollars a load. Profit is (12 − 5) × 7 = 49 hundred dollars a week; at 6 or 8 hundred loads it would be 48.
  4. The competitive benchmark. At P = MC, 19 − Q = 5 gives 14 hundred loads at 5 dollars, and customers' surplus would be 1⁄2 × 14 × 14 = 98.
  5. Where the surplus goes. Under the monopoly, customers keep 1⁄2 × 7 × 7 = 24.5, the owner takes 49 as profit, and 1⁄2 × 7 × 7 = 24.5 is the deadweight loss. Check: 24.5 + 49 + 24.5 = 98.
  6. The Lerner index. (12 − 5) ⁄ 12 = 7 ⁄ 12, so seven-twelfths of each load's price is markup over the cost of one more load.
  7. Say it in words. The laundromat washes half as many loads as a competitive market would, at a higher price, and 24.5 hundred dollars a week of value goes to no one.

Why it matters: Week 6's triangles are how the course measures what market power costs.

Worked example — two bookstores and free delivery (Week 12)

Two illustrative bookstores in one town each decide whether to offer free delivery. Monthly profit, counted in thousands and written (first store, second store), is (7, 7) if neither delivers, (9, 3) if only the first delivers, (3, 9) if only the second delivers, and (5, 5) if both deliver.

  1. The first store's best reply if the second does not deliver. Delivering pays 9 and not delivering pays 7, so it delivers.
  2. Its best reply if the second delivers. Delivering pays 5 and not delivering pays 3, so it delivers again. Delivering wins in both cases, so it is the first store's dominant strategy, and the second store's payoffs mirror the first's.
  3. The Nash equilibrium. Both deliver and earn 5 each. A store that dropped delivery alone would fall from 5 to 3, so neither moves.
  4. The dilemma. If neither delivered, each would earn 7, and together 14 instead of 10. From that cell either store gains 9 − 7 = 2 by starting delivery, so a promise to hold back would not last.
  5. Say it in words. Each store's own best choice leaves both earning less than they could together, which is the prisoner's dilemma, and customers get free delivery out of it.

Why it matters: compare within one column at a time; the biggest number in the table, 9, is not the equilibrium.

Worked example — a seedling nursery hires (Weeks 13 and 9)

An illustrative plant nursery is one of many, so it cannot change the price of a tray or the going wage. Its daily output runs 0, 8, 20, 30, 38, 44, and 48 trays of seedlings as it goes from no workers up to six.

  1. Marginal product. 8, 12, 10, 8, 6, and 4 trays.
  2. The dollar value at 4 dollars a tray. VMPL = P × MP: 32, 48, 40, 32, 24, and 16 dollars.
  3. Hiring at a 30-dollar wage. On the falling part, the 4th worker's 32 is at least 30 and the 5th worker's 24 is not, so the nursery hires 4. Revenue minus wages is 4 × 38 − 4 × 30 = 152 − 120 = 32 dollars, more than the 30 with 3 workers or the 26 with 5.
  4. The wage falls to 20 dollars. The 5th worker's 24 now clears the wage and the 6th worker's 16 does not, so the nursery hires 5: a movement along its labor demand curve.
  5. The tray price rises to 6 dollars, wage back at 30. VMPL becomes 48, 72, 60, 48, 36, and 24, so the 5th worker's 36 clears the wage and the nursery hires 5: labor demand has shifted right.
  6. Say it in words. A cheaper worker moved the nursery along its curve; dearer seedlings made every worker worth more and moved the whole curve.

Why it matters: the hiring rule is Week 9's marginal product turned into dollars, applied where marginal product falls.

Worked example — drone deliveries over a neighborhood (Weeks 14, 6, and 7)

Picture drone deliveries over one neighborhood (illustrative). Buyers' willingness to pay is P = 21 − Q ⁄ 3 and operators' marginal cost is P = 5 + Q ⁄ 3, with P in dollars a delivery and Q in deliveries an hour. Each delivery's noise costs the residents below 4 dollars, and they neither order nor fly the drones.

  1. The market. 21 − Q ⁄ 3 = 5 + Q ⁄ 3 gives 2Q ⁄ 3 = 16, so Q = 24 deliveries an hour at 21 − 8 = 13 dollars.
  2. Marginal social cost. MSC = (5 + Q ⁄ 3) + 4 = 9 + Q ⁄ 3.
  3. The efficient quantity. 21 − Q ⁄ 3 = 9 + Q ⁄ 3 gives 2Q ⁄ 3 = 12, so Q = 18 at 21 − 6 = 15 dollars.
  4. The deadweight loss. At 24 deliveries, MSC is 9 + 8 = 17 dollars and buyers value the last one at 13, a gap of 4. The loss is 1⁄2 × 4 × (24 − 18) = 12 dollars an hour.
  5. The corrective tax. A 4-dollar tax per delivery, the harm of one more, moves the market to 18. Buyers pay 15, operators keep 11, and the tax raises 4 × 18 = 72 dollars an hour. The total harm at 18 deliveries is also 72, but the tax is 4 dollars per delivery, not 72.
  6. Say it in words. Ignoring the noise, the market flies 6 deliveries an hour too many; a tax equal to the harm per delivery removes the 12-dollar loss instead of creating one.

Why it matters: the same tool that wasted surplus in Week 7 fixes a market with an externality.

Worked example — a bakery fan and a studio upstairs (Week 14)

An illustrative bakery's exhaust fan sends heat and noise into a dance studio upstairs, doing 250 dollars of harm a month. A quieter fan with a filter costs the bakery 180 dollars a month more.

  1. If the studio holds the right to quiet. The bakery compares 180 with the 250 it would owe, and 180 is less, so it installs the new fan.
  2. If the bakery holds the right to run its fan. The studio would gain 250 from the change, and the change costs 180, so any payment between 180 and 250 leaves both better off, and the new fan goes in.
  3. The gain. 250 − 180 = 70 dollars a month, whichever side holds the right.
  4. Say it in words. When the two can bargain cheaply, the right decides who pays, not whether the fan is replaced.

Why it matters: the Coase theorem needs clear rights and low bargaining costs; with many neighbors instead of one studio, bargaining gets costly and the result can fail.

Worked example — used sewing machines (Weeks 15 and 6)

Picture an illustrative resale trade in one model of sewing machine. Half the machines run well and half are lemons. Buyers would pay up to 1,200 dollars for a sound machine and up to 500 for a lemon. Owners of sound machines will not part with one for less than 1,050, and lemon owners will take 350.

  1. The uninformed offer. Buyers who cannot spot a lemon offer the average of their two values, (1,200 + 500) ⁄ 2 = 850 dollars.
  2. Who sells. 850 is below the 1,050 a good-machine owner needs, so those owners keep their machines; it is above a lemon owner's 350, so lemon owners sell.
  3. What buyers learn. Only lemons are on offer, so a buyer paying 850 gets a machine worth 500 and overpays by 350. Offers fall toward 500.
  4. The lost trade. Each good machine is worth 1,200 − 1,050 = 150 dollars more to a buyer than to its owner, a gain that never happens.
  5. The fix. A warranty from the seller (a signal) or a paid inspection by the buyer (screening) lets a good machine sell at a price between 1,050 and 1,200.
  6. Say it in words. Hidden quality before the sale drives good machines out of the market, which is adverse selection.

Why it matters: the timing test, hidden before the deal, is what separates this from moral hazard.

Worked example — an illustrative Lorenz curve (Weeks 15 and 1)

In an illustrative region, households sorted into fifths, from poorest to richest, get 6, 10, 16, 24, and 44 percent of all income.

  1. Cumulative shares. 6; 6 + 10 = 16; 16 + 16 = 32; 32 + 24 = 56; 56 + 44 = 100 percent.
  2. Read one point. The poorest 40 percent of households receive 16 percent of income, well below the 40 percent the line of perfect equality would give them.
  3. The area under the curve. With five strips each 0.2 wide, the area is 0.1 × (0.06 + 0.22 + 0.48 + 0.88 + 1.56) = 0.1 × 3.2 = 0.32.
  4. The Gini coefficient. 1 − 2 × 0.32 = 1 − 0.64 = 0.36. The same answer comes from the gap: (0.5 − 0.32) ⁄ 0.5 = 0.18 ⁄ 0.5 = 0.36.
  5. Sort the claims. "This region's Gini is 0.36" is positive: anyone can check the arithmetic. "This region's Gini is too high" is normative: it rests on a judgment about how much equality is enough.
  6. Say it in words. A Gini of 0.36 sits between 0, where every fifth gets a fifth, and 1, where one group gets everything: a moderate amount of inequality in this made-up region.

Why it matters: Week 15 measures inequality with Week 1's line between what is and what ought to be kept in place.

Worked example — who bears a parking tax (Week 7, carried into Week 14)

Picture the evening parking spots near a concert hall (illustrative): quantity demanded is Qd = 80 − 2P and quantity supplied is Qs = 4P − 10, with P in dollars and Q in spots a night.

  1. No tax. 80 − 2P = 4P − 10 gives 6P = 90, so P = 15 dollars and Q = 50 spots.
  2. A 6-dollar tax collected from lot owners. Supply becomes Qs = 4(P − 6) − 10 = 4P − 34, and 80 − 2P = 4P − 34 gives 6P = 114, so drivers pay 19 dollars, owners keep 13, and 42 spots are used.
  3. The burden. Drivers pay 19 − 15 = 4 dollars more and owners keep 15 − 13 = 2 dollars less, and 4 + 2 = 6.
  4. Why drivers carry more. Drivers change their quantity by 2 spots for each dollar, and owners change theirs by 4, so the less responsive side, the drivers, bears more of the tax, though the owners write the check.
  5. Revenue and loss. The tax raises 6 × 42 = 252 dollars a night, a transfer, and the deadweight loss is 1⁄2 × 6 × (50 − 42) = 24 dollars a night.
  6. Say it in words. Most of the tax lands on the drivers, and 8 spots a night that were worth using go empty.

Why it matters: responsiveness, the Week 5 idea, decides incidence, and the same deadweight-loss triangle returns in Week 14.

Self-check questions

Try each one, then read the check and the one-line why. Every case is illustrative.

  1. A smoothie bar's output rises from 70 to 80 cups an hour when it adds a fifth worker. What is that worker's marginal product? Check: 80 − 70 = 10 cups. Why: marginal product is the change in output, not the output per worker.
  2. A ceramics kiln costs a potter 96 dollars a week whether or not she fires it. What is her average fixed cost at 8 pots, and what happens to it at 12? Check: 96 ⁄ 8 = 12 dollars a pot, and it falls to 8 at 12 pots. Why: the same fixed cost is spread over more output.
  3. A firm's cost per unit (ATC) is 20 dollars at 4 units, and the 5th unit adds 15 dollars. What is ATC at 5 units? Check: (20 × 4 + 15) ⁄ 5 = 95 ⁄ 5 = 19 dollars. Why: a marginal cost below the average pulls it down.
  4. A price-taking firm's AVC bottoms out at 9 dollars and its ATC bottoms out at 13. The market price is 11. Does it operate this week, and does it earn economic profit? Check: it operates, at a loss. Why: 11 covers average variable cost but not average total cost.
  5. The same firm's market price falls to 8. What does it do? Check: it shuts down and loses only its fixed cost. Why: 8 is below the lowest average variable cost of 9.
  6. Fresh-pasta makers in one city are price takers, and they earn economic profit. Which curve moves in the long run, which way, and what happens to the price? Check: market supply shifts right as firms enter, and the price drops to the bottom of the ATC curve. Why: entry continues until economic profit is zero.
  7. A seller with demand P = 16 − Q (Q in hundreds) can sell 3 hundred at 13 dollars or 4 hundred at 12 dollars. By how much does revenue change, per unit, as it moves from 3 hundred to 4? Check: 12 × 4 − 13 × 3 = 48 − 39 = 9 dollars a unit. Why: selling more means cutting the price on every unit, so marginal revenue is below the 12-dollar price.
  8. A software firm's student license costs 25 dollars, while one more copy costs it 15. What is its Lerner index, in words? Check: (25 − 15) ⁄ 25 = 2 ⁄ 5: two-fifths of the price is markup. Why: the index is the markup over marginal cost as a share of the price.
  9. That firm sells the same license for less to anyone with a student ID and blocks resale. Which three conditions of price discrimination does the case show? Check: market power, a way to tell buyers apart by what they will pay (the ID), and a way to stop resale. Why: the price gap has nothing to do with cost.
  10. In a fresh pricing game, keeping a deal pays a firm 8 and cheating while the rival keeps it pays 11. What is the gain from cheating, and why does it matter? Check: 11 − 8 = 3. Why: that temptation is why collusion is unstable, besides being illegal.
  11. A dog-walking service in a city full of rivals earns economic profit this year. What happens to its own demand curve over time? Check: it shifts left as new services open, until P = ATC. Why: free entry competes the profit away, even with differentiated services.
  12. A farm sells peppers at 3 dollars a pound, and its 3rd picker adds 12 pounds a day. What is that picker's value of marginal product, and is the farm right to hire her at a 40-dollar daily wage? Check: 3 × 12 = 36 dollars a day, so no: she adds less than she costs. Why: compare dollars with dollars, assuming marginal product is already falling there.
  13. Clinics see more patients and want more X-ray technicians at every wage, with no other change. Name the curve, the direction, and where the wage and the number employed go. Check: labor demand shifts right; the market wage goes up and more technicians are hired. Why: the demand for technicians is derived from the demand for care.
  14. Each tree a homeowner plants gives neighbors 3 dollars of benefit in shade and cleaner air. Does the market plant too many trees or too few, and what is the corrective policy? Check: too few; a 3-dollar subsidy per tree. Why: a positive externality means buyers ignore part of the benefit.
  15. Sort four goods: a flood levee protecting a whole town, fish in a lake open to anyone, a rarely crowded members-only pool, and a slice of pizza. Check: public good, common resource, club good, private good. Why: ask rival and excludable for each one.
  16. 250 households each value a new park bench path at 4 dollars, and it costs 700. Is it worth building, and why might it go unbuilt? Check: 250 × 4 = 1,000 is more than 700, a net gain of 300; free riders may not chip in. Why: no one can be kept off the path.
  17. After buying a rental-car damage waiver, a driver takes rough back roads he would otherwise avoid. Which problem is this? Check: moral hazard. Why: the hidden action comes after the deal.
  18. A seller of used phones knows each battery's wear, and buyers cannot check it. Which problem, and name one market fix. Check: adverse selection; a warranty (a signal) or a battery test (screening). Why: the hidden trait exists before the sale.
  19. A plan to start a language course "next month" keeps slipping. Which pattern is this, and is it a sign of foolishness? Check: present bias, a predictable pattern, not foolishness. Why: the cost now weighs more than the same cost later.
  20. The area under an illustrative Lorenz curve is 0.35. Find its Gini coefficient. Check: 1 − 2 × 0.35 = 0.30. Why: the farther the curve sags, the smaller the area and the larger the Gini.

Check yourself — choose one. Write three or four sentences to a friend.

  • (a) Pick one of the term's named debates (antitrust, the minimum wage, carbon pricing, or inequality), state one positive claim and one normative claim from it, and give each side's strongest point without saying which side is right.
  • (b) Pick one market failure from Weeks 11, 14, or 15 and explain, in plain words, why the market alone misses the efficient outcome and which tool from the course could help.

Either one rehearses an explain-why skill the final checks.

Watch before the exam

These fourteen videos are already linked on the course's Weeks 9–15 readings pages, and every one has captions. Use them to review and to connect weeks, not to learn anything new.

Study plan

A short countdown. Count back from the day you plan to sit the final.

  • Two weeks out: as you finish Week 14, start a one-page list of the second half's swapped pairs. Begin with marginal and average cost, accounting profit against economic profit, and the shutdown and break-even prices. Add P = MC and MR = MC, the price read from demand and from the MR line, and profit and deadweight loss. Add the Nash equilibrium and the cooperative outcome, marginal product and its dollar value, and a movement along labor demand and a shift of it. Add private and social cost and a per-unit tax and a total harm. When Week 15 arrives, add adverse selection and moral hazard, and a Gini of 0 and of 1.
  • One week out: as you finish Week 15, reread the Week 10 topic review and redo the banner-shop worked examples. Week 10 holds 7 of the final's 40 questions, and the practice exam has only 3 from it.
  • At the start of Week 16: take the practice exam once, cold. Note which objectives cost you the most, and remember that it reaches back to Weeks 1–4 more than the final does.
  • Three or four days before you sit it: reread the topic reviews for your two weakest objectives and redo the matching worked examples from scratch without looking. Give Weeks 11 and 14 a second pass, since they hold 6 questions each.
  • Two days before: run the paired exam-prep tutorial, the copy-and-paste chatbot tutor or its no-AI route, for active recall. Tell it which areas the practice exam exposed.
  • The day before: retake the practice exam, explain every miss out loud, and work the self-check questions above. Then skim the Weeks 1–8 callback section once, so the early tools are fresh when they appear inside a later week.
  • Right before: skim the "predictable mistakes → cures" lines for Weeks 9 to 15. Then stop studying, and take a few slow breaths.

This guide is for review. The exam-prep tutorial and the practice exam are where you rehearse. Pulling ideas back out of memory, not rereading, is what makes them stick.

Exam format

Every one of the 40 questions counts the same, and the final checks your own understanding without AI. The questions come one at a time, and there is no going back, so settle each one before you move on.

Test-taking strategies for this material:

  • Name the market structure first. Before you apply a rule, ask whether the seller takes the price or faces a downward-sloping demand. Every seller uses MR = MC; only the price taker's version reads P = MC.
  • Name the curve, the direction, and the new outcome. For any change, decide whether it is the good's own price (a movement along) or something else (a shift), then say where the price and quantity, or the wage and the number employed, land.
  • Write the table out. For a cost, product, revenue, or hiring table, write the marginal row on scratch paper by subtracting each total from the next before you choose.
  • Keep marginal and average apart. Marginal answers "what does one more add?"; average answers "what is it per unit so far?" Many wrong options swap the two.
  • Draw the triangle. For a deadweight loss, find the base (the units lost or added in error) and the height (the gap at the end), then take one half of their product. Revenue and profit are transfers, not losses.
  • Check the timing. For hidden information, ask whether the hidden thing existed before the deal (adverse selection) or happened after it (moral hazard).
  • Sort the claim. For a policy statement, ask whether evidence could check it (positive) or whether it says what ought to happen (normative), whichever side it favors.
  • Say the result in words. After any calculation, say in one sentence what the number means for buyers, sellers, workers, or bystanders; it catches slips before you commit.

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