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Week 12 · Module overview

Week 12 — Confidence Intervals for a Proportion

Introduction to Statistics Free sample edition

Welcome to your Week 12 Start Here page.

Why this matters

A public-health office reports the share of residents who are vaccinated, and a news poll reports the share of voters who back a measure. Both report one share plus a margin of error, and both need to be clear about what bias that margin can still hide.

In plain language

Many of the numbers you hear about the world are shares, whether they come from 1,000 voters, 600 clinic patients, or 333 penguins. Think of the share of voters who back a plan, the share of patients who got a flu shot, or the share of penguins that are female. This week you learn to report a share from a sample as a confidence interval for a proportion (a range of believable values for the true share in the whole group, with a stated level of trust). You will build it by hand and with a spreadsheet, and you will learn to read the margin of error (the plus-or-minus part of the range) that comes with every news poll. You will also learn to plan how many people a survey needs, and what the plus-or-minus leaves out.

  • Already covered: Week 10 showed how a sample proportion wobbles; Week 11 built a confidence interval for a mean.
  • This week: the one-proportion z interval, poll margins of error, sample sizes, and bias.
  • How they connect: the same estimate-plus-or-minus recipe, now built around a share.
  • Where we're going next: Week 13 turns to testing a claim.

Focus

How far can you trust a share from a sample, and what does its plus-or-minus leave out?

By the end of this module, you should be able to:

  • [ ] check the conditions and build a 95% interval for a proportion
  • [ ] interpret the interval and say which values are plausible
  • [ ] read a poll's margin in percentage points and plan a sample size
  • [ ] name the biases a margin of error does not cover

To-do

Plan your week

  1. Read Chapter 12 (about 45 minutes), then the lecture and slides.
  2. Work the lecture tutorial (60–90 minutes) and the practice set (about 20 minutes).
  3. Do the Data Lab (about 90 minutes), the discussion, and the assignment.
  4. Take the quiz.

This week's checkpoint: build the 95% interval for the share of female penguins and say whether an even split is plausible.

You've got this

Take it in order, and ask your instructor early if a step snags.

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