Mind Blown

Aggregation

squashing groups into one number. Simpson's paradox = when doing so reverses the real trend.

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The Statistic That Reverses ItselfMind Blown

More terms from this board

Confounder (lurking variable)a hidden factor (stone size, department) that skews BOTH the groups and the outcome.
Weighted averagea total where big groups count more; unequal groups let a subgroup winner lose the pooled number.