The big idea
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An average without a spread is half a description
And usually the less useful half.
Statistics for Everyday Life · Lesson 5
Variation and Spread
beginner⏱ 17 min · 7 cards
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Same Average, Different Story
“Route A: 28, 29, 30, 31, 32 minutes.”
Both average 30 minutes. Would you commit to a 35-minute guarantee?
Spread, not the average, determines what you can commit to.
Decide which situations you could actually commit to.
Key term
The illusion that fools everyone
Why does the worst-performing group improve after you intervene?
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Regression to the mean
Extremes are partly luck, and luck does not repeat. The next measurement is likely to be less extreme whatever you did — which is why control groups exist.
Flight school
Praise, criticism, and a wrong lesson
Instructors saw trainees do worse after praise for a great landing, and better after criticism for a poor one. They concluded criticism works.
Regression to the mean predicts exactly this pattern with nothing said at all. The memorable lesson was the mistaken one.
Nuance
Careful
Common misconception
Low variation is not automatically good. Consistently near the wrong target is precise and inaccurate — and it looks reliable.
Quick check
What makes statistics necessary?
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