Simpson's paradox
Simpson's paradox is the reversal that occurs when a trend present in every subgroup disappears or flips once the subgroups are pooled, because a lurking variable is unevenly spread across the groups.
See it move
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In Region 1, Campaign A converts 20% versus Campaign B's 16%. In Region 2, Campaign A converts 90% versus 85%. A wins both regions individually. But pooled, Campaign A converts 85÷250 = 34.0% while Campaign B converts 178÷250 = 71.2% — B wins overall, because A's leads were concentrated in the lower-converting region.
Where it fits
TopicDescriptive StatisticsAdvancedSubjectData Analysis & StatisticsAdvancedTopicRegression Diagnostics & ProblemsAdvanced