COVID-19: Making Excess Death Comparisons
May 22, 2020
Jason Oke, Carl Heneghan
A lot of has been made of the excess mortality in the current COVID outbreak. To estimate the excess mortality, a count of deaths from all causes relative to what would normally be expected is required.
The usual way to create excess mortality is to compare with the five-year average.
Another way is to compare with a year where excess deaths occurred. In the 2017 to 2018 winter period, there were 50,100 excess winter deaths compared with the five year average of the preceding years in England and Wales.
The difference in all-cause deaths between 2017/18 and the five-year average might not look like much, but they have a dramatic effect on the excess difference.
Comparing the number of deaths in the last year to the five year average we observe an excess of 53,240 deaths.
However, comparison with 2017/18 shows an excess of 32,720 deaths (20,520 fewer than the five-year average comparison).
Context is helpful when making comparisons. In some years excess deaths will seem better or worse depending on the comparator used – comparing with the year with the least deaths, for instance, would inflate the excess.
What looks like small differences over the year can accumulate and give rise to considerable numbers of extra deaths, which also varies depending on the timing of when they are assessed.
Jason Oke is a Senior Statistician at the Nuffield Department of Primary Care Health Sciences and Module Coordinator for Statistical Computing with R and Stata (EBHC Med Stats), and Introduction to Statistics for Health Care Research (EBHC), as part of the Evidence-Based Health Care Programme.
Carl Heneghan is Professor of Evidence-Based Medicine, Director of the Centre for Evidence-Based Medicine and Director of Studies for the Evidence-Based Health Care Programme. (Full bio and disclosure statement here)
Disclaimer: the article has not been peer-reviewed; it should not replace individual clinical judgement, and the sources cited should be checked. The views expressed in this commentary represent the views of the authors and not necessarily those of the host institution, the NHS, the NIHR, or the Department of Health and Social Care. The views are not a substitute for professional medical advice.