Simpson’s paradox occurs when an association or trend changes, disappears, or reverses after data is grouped by another variable. It warns against treating correlation as causation: a confounder can make an overall comparison point in the wrong direction. section 2.0 section 2.1
Kidney stone treatments
Overall, treatment B appears better, with an 81% recovery rate versus 78% for A. After separating patients by stone size, A performs better for both small stones (93% versus 87%) and large stones (73% versus 63%). Stone size changes the conclusion because treatment groups contain different mixes of small and large stones. section 2.1
Table 2.1 Kidney stone dataset
|
Treatment
|
Size
|
Patients
|
Recovered
|
|---|---|---|---|
|
A
|
Small
|
87
|
81
|
|
B
|
Small
|
270
|
234
|
|
A
|
Large
|
263
|
192
|
|
B
|
Large
|
80
|
50
|
Inspect the kidney stone counts and compare the unstratified recovery table with the size-specific rates to see how aggregation reverses the apparent treatment ranking. section 2.1
Use the paradox as a practical check before making a causal decision:
- Ask which variables affect both the choice or treatment and the outcome; these are potential confounders. section 2.0
- Compare results within relevant groups instead of relying only on aggregated averages or correlations. section 2.7
- Use domain knowledge and causal graphs to decide which variables belong in an adjustment analysis. section 2.9
- In regression, include the confounder when appropriate: the unadjusted price model suggests a positive customer trend, while including distance estimates the intended effect as about −1.99. section 6.1.1

