According to CBS’ definition, excess mortality occurs when the observed number of deaths over a particular period is higher than the number that is expected for that same period.
As of 2 October 2020, the following method is being implemented by CBS:
The expected mortality - in the absence of an epidemic - 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 week over that week and the six surrounding weeks. These average weekly mortality figures 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 2020, which stands at 153,402 deaths. For 2020, this is 153,402 and for 2021 this is 154,887. The number for 2020 has been derived from the Key Forecast 2019-2060 and the number for 2021 from the Population Forecast 2020-2070 (excluding an assumed excess number of deaths due to the coronavirus pandemic). The margins around expected mortality are estimates, based on the observed spread of weekly mortality over the past five years.
CBS implementing a different method as of 2 October 2020
Expected mortality had no coronavirus epidemic occurred has been estimated based on the number of deaths in the preceding weeks, adjusted for seasonal factors. It is assumed that the expected weekly mortality from week 11 onwards was the same as in weeks 3 to 10 inclusive. Adjustments for the average weekly temperature have been made as part of the seasonal factors. The assumption is that every one degree Celsius above the average weekly temperature in weeks 3 through 10 results in 1 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 the minor temperature differences relative to weeks 3 to 10. From week 15 onwards, it was warmer and the corrections amounted to several percentage points. As of week 25, the maximum temperature exceeded 16.5 degrees Celsius (with the exception of week 28). The average temperature fell below 16.5 degrees again as of week 35.
Reason for this changed method
Expected mortality as calculated according to the method that was in use prior to 2 October 2020 relied heavily on the 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, but the method was not robust enough to determine excess mortality over a longer period of time. That is why, as of 2 October 2020, CBS uses a long-term average to serve as the basis for expected mortality. This will include a range indication of the commonly determined margins around weekly mortality. These have been estimated based on the range distribution in previous years.