“Kievit et al. (2013). Simpson’s paradox in psychological science: A practical guide.” - Article summary

Simpson’s paradox states that the direction of an association at the population-level may be reversed within subgroups of that population. Inadequate attention to the Simpson’s paradox may lead to faulty inferences. The Simpson’s paradox can arise because of differences in proportions on subgroup levels compared to population levels. It also states that a pattern (association) does not need to hold within a subgroup.

The paradox is related to a lot of things, including causal inference. A generalized conclusion (e.g. extraversion causes party-going) might hold for the general population, but does not mean that this inference can be drawn at the individual level. A correlation across the population does not need to hold in an individual over time.

In order to deal with Simpson’s paradox, the situations in which the paradox occurs frequently have to be assessed. There are several steps in preventing Simpson’s paradox:

1. Consider when it occurs.
2. Explicitly propose a mechanism, determining at which level it is presumed to operate.
3. Assess whether the explanatory level of data collection aligns with the explanatory level of the proposed mechanism.
4. Conduct an experiment to assess the association between variables.

In the absence of strong top-down knowledge, people are more likely to make false inferences based on Simpson’s paradox.