Title: Week 12 Overview — Start Here
Course: Introduction to Statistics (18-Week Generic Edition)

Welcome to Week 12: Confidence Intervals for a Proportion

This is your home base for the week. Read it first, then work the checklist below from top to bottom. Everything you need is linked inside the module.

Last week you built your first confidence intervals — for a mean, with the t-multiplier. This week the same machine estimates the most public statistic in the world: the percent. Every poll in your feed ends with the same fine print — "margin of error ±3 percentage points" — and by the end of this week that fine print is yours: you'll know where the 3 comes from, you'll recompute it from a real poll's own numbers in the lab, and you'll know the two things it does not protect you from.

The week's big question

"Every poll ends with '±3 points.' Where does that number come from, what does it promise — and what does it quietly leave out?"

By the end of this week, you can…

Use this as a checklist. If you can do all five out loud, you're ready for the quiz.

- [ ] Compute p̂ and its standard error — SE = √( p̂(1 − p̂) ⁄ n ) — feeding it proportions, never counts.
- [ ] Check the password before building — random sample · at least 10 successes and 10 failures · population at least 10 × n.
- [ ] Build the one-proportion z-interval — p̂ ± z·SE with z = 1.645 / 1.96 / 2.576 — and state the trade-off: surer means wider.
- [ ] Interpret it correctly — "we are 95% confident the interval captures the true proportion" — and spot the two classic misreadings (it's never a 95% chance about one interval, and never about 95% of individuals).
- [ ] Price a survey — n = p(1 − p)(z ⁄ ME)², with p = 0.5 when you know nothing, always rounded up — and audit a media poll's margin of error from its own n.

What to do this week, in order

The table below lists the week's items in working order, with what each is worth and when it's due.

# | Do this | Type | Due
1 | Read Chapter 12 — the module's primary reading (the z* table lives here) | Chapter (ungraded prep) | Early in the week
2 | Skim the slides (Deck 12) and the Week 12 lecture outline; browse the Readings & Resources links that interest you | Prep (ungraded) | Alongside class
3 | Lecture Tutorial 12 — work through the week's ideas with your chatbot, then submit the share link + Completion Summary | Tutorial · graded (Lecture tutorials, 20% group) | End of Week 12
4 | Practice exercises — quick reps with the AI coach | Practice · ungraded | Before the quiz (recommended)
5 | Data Lab 12 — "Anatomy of a Poll: Audit the ±" — recompute a real (or provided) poll's margin of error in six spreadsheet cells | Data lab · graded (Data labs, 15% group) | End of Week 12
6 | Quiz 12 — p̂ and SE, conditions, building and reading intervals, sample size, the margin of error in the media | Quiz · graded (Quizzes, 15% group) · closed to AI | End of Week 12
7 | Discussion 12 — "The 400 vs. 18,000 Problem" — a random 400 says 62%, an opt-in 18,000 says 44%; argue out which one the council should trust, post the AI summary + chat link | Discussion · graded (Discussions, 15% group) | Initial post two days before week's end; replies by end of Week 12
8 | Assignment 12 — "The Honest Percent" — four AI-coached problems; submit the report (score on line 1) + chat link | Assignment · graded (Assignments, 25% group) | End of Week 12

Heads-up on the AI work: in this course the chatbot drafts, and you judge. This week's classic chatbot fumbles: answering the famous "1,067" for a ±3 sample size instead of rounding 1067.11 up to 1,068, and blessing the sentence "there's a 95% chance the true value is in the interval" (the 95% belongs to the method, not to one interval). Catching these is literally the lab's graded AI-critique step.

Late policy reminder: 10% off per day late. If life happens, reach out to your instructor before the deadline — early is always easier.

How to succeed this week

- Say the chant. Estimate ± multiplier × standard error — "center ± reach." It's the same anatomy as last week; only the fuel changed (means → t, proportions → z).
- Memorize three numbers, not a table. 1.645 (90%) · 1.96 (95%) · 2.576 (99%). Everything else this week is arithmetic.
- Check the password before you build. Random · ≥10 successes and ≥10 failures · population ≥ 10n. A failed condition isn't a technicality — it's the machine telling you the answer would be junk.
- Practice the sentence. Every interpretation needs all three ingredients: the confidence level, the interval, and the population's proportion. If your sentence contains "chance" or "of the people," rebuild it.
- Read whole intervals, not point estimates. 48% ± 3 runs from 45 to 51 — that's "majority undecided," not "the bond fails." This one habit will change how you read the news.

You already own every ingredient this week uses — p̂ from Week 1, the normal curve from Week 8, standard error from Week 10, and interval thinking from Week 11. This is the week they click together in public.