According to CBS’ definition, excess mortality occurs when the observed number of deaths over a particular period is higher than the number expected for that same period.
CBS has implemented the following method as of 2 October 2020
Expected mortality, had no coronavirus epidemic occurred, has been estimated based on the observed number of deaths over the years 2015-2019. First, mortality on a weekly basis is established for each of the preceding years. Subsequently, an average mortality figure is determined for each consecutive week and the six surrounding weeks. These weekly averages offer an approximation of the expected weekly mortality, as the trend growth of population ageing has not yet been taken into account. For this reason, the weekly mortality figures were rescaled to fit the expected annual mortality over the year. For 2020, this is 153,402 and for 2021 the number is 154,887. The number for 2020 is taken from the ‘Kernprognose 2019-2060’ and the number for 2021 from the ‘Bevolkingsprognose 2020-2070’ (excluding the assumption of excess deaths due to the coronavirus epidemic). The margins around expected mortality are estimates, based on the observed spread of weekly mortality over the same five years. This method has been applied retrospectively to the expected mortality from week 1 in 2020.
CBS used a different method before 2 October 2020
Expected mortality, had no coronavirus epidemic occurred, had been estimated based on the number of deaths in the preceding weeks, adjusted for seasonal factors. As of week 11, it is assumed that the expected weekly mortality is the same as in weeks 3 to 10. The seasonal factors are adjusted for the average weekly temperature. The assumption is that every one degree Celsius above the average weekly temperature in weeks 3 through 10 results in one percent decline in mortality. Once the maximum temperature rises above 16.5 degrees Celsius, mortality rises again by 2 percent per degree Celsius. For weeks 11 to 14, the correction was very small due to little difference in temperature compared to weeks 3 to 10. From week 15 onwards, it was warmer and the corrections amounted to several percentage points. The maximum temperature has exceeded 16.5 degrees since week 25 (with the exception of week 28). The average temperature fell below 16.5 degrees again as of week 35.
Reason for this change
The expected mortality according to the method that was in use until 2 October 2020 relied heavily on observation of mortality figures over weeks 3 to 10 inclusive of 2020. It was a sound method for mortality during the first wave of the coronavirus epidemic; however, it is not robust enough to determine excess mortality over a longer period of time. This is why, as of 2 October 2020, CBS has implemented a long-term average to serve as a basis for expected mortality. This will include a range indication with the margins within which weekly mortality is usually found, estimated based on the distribution in previous years.