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Data & Analytics · Advanced

Simpson's Paradox

A phenomenon in which a trend in several groups reverses or disappears when the groups are combined.

Definition

Simpson's paradox is caused by unequal group sizes interacting with a lurking variable, and it is a reminder to analyze data at the right level of aggregation.

Example

Treatment A has a higher success rate in every hospital but a lower overall rate because it was used on harder cases.

Exam tip

Aggregated results that contradict subgroup results point to Simpson's paradox.

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