Title: Welcome to Week 4 — two variables at a time
Course: Introduction to Statistics (18-Week Generic Edition)

Hi everyone,

Nice work last week — you can now take any single column of data and tell its whole story: shape, center, spread, and the outliers that distort it. This week, the natural next question: what happens when two columns move together?

This week — Relationships Between Two Variables — we tackle the big question: When one thing moves, does the other move too — and how do we measure that link without calling it a cause? You'll read scatterplots like sentences (direction, form, strength), compress a whole cloud of dots into one number (the correlation r), untangle two kinds of percent inside two-way tables — and learn why even a gorgeous correlation never proves an arrow.

Three things not to miss: 1. Chapter 4 is your primary reading — start there; the cocoa-stand and aquarium examples in it come back everywhere this week. 2. Data Lab 4 brings the penguins back for their best act yet: flipper length vs. body mass in a real scatterplot, your first =CORREL(), and a species-by-island table with a lurking variable hiding in plain sight. Due at the end of Week 4. 3. Discussion 4 asks whether your screen time causes your worst sleep — or the other way around, or neither. Your initial post is due two days before the week ends, so classmates have time to reply.

One heads-up: this is the week the course's favorite slogan — correlation is a handshake, not a push — stops being a slogan and becomes a skill. By the end of the week you'll spot the third hand behind a headline in seconds.

Open the Start Here / Module Overview page first — it lays out everything in order with due points. Bring a hunch about two things that move together; we'll measure it.

See you in the course!