Week 4 — Exploring Relationships Between Two Variables

Welcome to your Week 4 Start Here page.

Why this matters

Two numbers that move together show up everywhere. A public-health worker comparing hot days with hospital visits and a small-business owner comparing ad spending with sales both need to describe the pattern honestly.

In plain language

Some numbers seem to move together, like hot days and cold drinks sold, and this week you learn to check whether they really do and how closely. You will draw a scatterplot (a graph with one dot for each individual, placed by its two measurements). You will read a correlation (one number that says how tightly the dots follow a straight line). You will also read a table that sorts people or animals two ways at once. Last, you will learn why "these move together" is not the same as "one causes the other."

• Already covered: Week 1 separated observational studies from experiments; Week 2 drew one variable at a time and introduced the two-way table; Week 3 put numbers on center and spread.
• This week: two variables at once — scatterplots, the correlation r, marginal and conditional distributions, and the difference between association and causation.
• How they connect: a scatterplot is a dot plot with a second axis, and a conditional distribution is a relative-frequency table for one slice of a two-way table.
• Where we're going next: Week 5, Probability Foundations, opens a new unit on chance and reads probabilities straight out of the same two-way table.

Focus

Do two variables move together, and is it cause? You plot pairs, read r, and read a table two ways.

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

• [ ] describe a scatterplot by direction, form, strength, and unusual points
• [ ] read r and say what it can and cannot measure
• [ ] read marginal and conditional distributions from a two-way table
• [ ] name a lurking variable behind an association

To-do

Plan your week

1. Read Chapter 4 (about 35 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: say what share of the Biscoe penguins are Gentoo, and which total you divide by.

You've got this

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

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