The big idea
📉
One number, four very different pictures
A correlation coefficient is a summary of a scatter plot. Ask to see the plot.
Statistics for Everyday Life · Lesson 2
Correlation and Regression
beginner⏱ 18 min · 7 cards
Try it
Read the Scatter
“A clean oval cloud of points sloping upward.”
Does a single correlation coefficient capture this relationship?
Decide when a single coefficient describes the relationship and when it misleads.
Key term
The word that carries everything
What does r near zero actually mean?
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No linear relationship
Not "no relationship". A perfect inverted-U — like sleep and performance — can give r near zero while being entirely predictable.
The confident straight line
Fitted to 800, asked about 3,000
Stores from 200 to 800 square metres show £3,000 of revenue per extra square metre.
Extrapolated to a 3,000-square-metre store the line promises a fortune — answering a question it was never in a position to answer.
Nuance
Careful
Common misconception
A high R² does not mean good predictions for individuals. It describes variation explained across the group; people still scatter widely around the line.
Quick check
Why can a random sample of 1,000 represent millions?
You did it
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