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Learn the skill
When two groups differ, ask how the groups were formed and what else differed. A measured association can be real while its explanation remains uncertain. The classroom studies here are fictional.
Worked example
If volunteers choosing a guide score higher, the guide might matter, but prior interest could also differ. Choice alone does not isolate a cause.
Original fiction: The Optional Practice Sheet
A fictional class offered an optional practice sheet before a quiz. Students who chose it later had a higher average quiz score. They were not randomly assigned, and their earlier scores were not recorded in the comparison. The teacher notes that motivated students might have been more likely to choose practice. The record supports an association but does not isolate the sheet's effect.