Simpson's paradox in Covid-19 case fatality rates: a mediation analysis of age-related causal effects
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We point out an example of Simpson's paradox in COVID-19 case fatality rates (CFRs): comparing data from >72,000 cases from China with data from Italy reported on March 9th, we find that CFRs are lower in Italy for each age group, but higher overall. This phenomenon can be explained by a stark difference in case demographic between the two countries. Using this as a motivating example, we review basic concepts from mediation analysis and show how these can be used to quantify different direct and indirect effects when assuming a coarse-grained causal graph involving country, age, and mortality. As a case study, we then investigate how total, direct, and indirect (age-mediated) causal effects between China and Italy evolve over two months until May 7th 2020.
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