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Simpson's paradox

As explained in Statistics Slam Dunk

Simpson's paradox is a statistical phenomenon in which an association observed in aggregated data reverses or conflicts with the associations found in relevant subpopulations.

Simpson's paradox occurs when an association in aggregated data conflicts with the associations found inside relevant subpopulations. A single overall correlation can therefore conceal a different pattern within meaningful groups. section 10.4.1

Consider NBA team payrolls and regular-season wins. Across the full 2012–13 data, payroll and wins have a positive association; after separating teams by season-ending result, the relationships are flat. Coloring the groups makes the hidden structure visible. section 10.4.1

Reveal the paradox by treating aggregation as an analytical choice:

  • Identify relevant subpopulations, such as playoff, non-playoff, and championship outcomes. section 10.4.1
  • Recalculate or visualize each subgroup separately rather than trusting the pooled result. section 10.4.1
  • Compare the directions and strengths of the pooled and subgroup relationships before drawing a conclusion. section 10.5

Payroll, wins, and season outcomes

A pooled NBA analysis reports a positive payroll–wins correlation, while grouping teams by season-ending factor produces flat within-group relationships. The result warns against interpreting one aggregate trend as the experience of every group. section 10.4.1 section 10.5

How Statistics Slam Dunk covers it

How deep it goes
Introduces the phenomenon briefly and demonstrates visual detection, without developing a formal proof or causal explanation.
Examples
Uses NBA team payrolls, regular-season wins, playoff factors, and the 2012–13 season.
What you can do after
Diagnose/debug with it: compare pooled and subgroup correlations before interpreting an apparent trend.
How it connects
Connects correlation, subpopulations, data visualization, grouping, and regular-season wins.

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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Statistics Slam Dunk. The section links point to the parts of the book it draws on.