Simpson's paradox in Covid-19 case fatality rates: a mediation analysis of age-related causal effects
We point out an instantiation of Simpson's paradox in Covid-19 case fatality rates (CFRs): comparing data of 44,672 cases from China with early reports from Italy (9th March), we find that CFRs are lower in Italy for every age group, but higher overall. This phenomenon is explained by a stark difference in case demographic between the two countries. Using this as a motivating example, we introduce 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 total, direct, and indirect (age-mediated) causal effects between different countries and at different points in time. This allows us to separate age-related effects from others unrelated to age, and thus facilitates a more transparent comparison of CFRs across countries throughout the evolution of the Covid-19 pandemic.