Calculation and meaning of "excess mortality": A comparison of Covid- and pre-Covid mortality data in 31 Eurostat countries from 1965 to 2021.

Determining ''excess mortality'' makes it possible to compare the burden of disasters between countries and over time, and thus also to evaluate the success of mitigation measures. However, the debate on Covid-19 has exposed that calculations of excess mortalities vary considerably depending on the method and its specification. Moreover, it is often unclear what exactly is meant by ''excess mortality''. We define excess mortality as the excess over the number of deaths that would have been expected counter-factually, i.e. without the catastrophic event in question. That is, we include all normally occurring flu and heat waves, which are excluded by some authors with the consequence that they almost always record low expected values and correspondingly high excess mortality rates. Based on this definition, we use a very parsimonious calculation method that is easy to understand even for laypersons, namely the linear extrapolation of death figures from previous years to determine the excess mortality during the Covid-19 pandemic. But unlike other literature on this topic, we first evaluated and optimised the specification of our method using a larger historical data set in order to identify and minimise estimation errors and biases. The result shows that the excess mortality rates continuously published by international statistical offices -- OECD and Eurostat -- are often inflated and would have exhibited considerable excess mortalities in many countries and periods before Covid-19, if this value had already been of public interest at that time. It also reveals that mortality rates already fluctuated strongly in the past and that in a third of the countries studied, individual values from the past exceed the current fluctuations due to the Covid-19 pandemic. Three conclusions can be drawn from this study and its findings: 1) All calculation methods for current figures should first be evaluated against past figures. 2) The definition of excess mortality used should be made explicit. 3) Statistical offices should provide more realistic estimates.