Posted by Roger Mallett Posted on 3 March 2024

How the ONS shrank the excess death figures

LAST week (February 20) the Office for National Statistics (ONS) published new estimates of excess deaths in the UK based on a revised methodology. The figures were greeted with some scepticism. Academics from the University of Oxford immediately warned that the new modelling revealed a major drop in expected deaths in 2020, ‘making it appear that far more people had died than normal during the first year of the pandemic’, as the Telegraph reported. The new modelling was also criticised for revising down excess deaths last year, ‘even though many charities and universities have reported unusually large upswings in mortality rates for conditions like heart disease‘. Indeed.

I too wondered how the ONS managed to ‘revise down’ excess deaths in 2023 and make them look so much fewer, so I decided to I run their model(s) independently and to conduct an independent re-analysis. What follows is that – and an explanation of how the trick is performed!

First I went to the link that describes the ONS methodology; then to this link which is the corresponding dataset; and finally to this link which is their R-based code.

I began by assuming the ONS dataset itself is reliable. However, it is possible to question the total population estimates within that dataset, especially in respect of immigration, including illegal immigration. Consequently, there may be a degree of underestimation of excess deaths on that basis. Nevertheless, the key issue relates to how the data are processed to provide ‘excess deaths’.

Giving public access to their code is unusual. Probably the ONS anticipated being deluged with hard questions from statistically competent people, and thought this is the best way of addressing such queries. This is welcome and sensible. However, I decided to use an independent code.

The key issue, as noted above, is that the new methodology estimates far smaller numbers of excess deaths in 2023 (though less so in 2020 and 2021, and actually rather more in 2022). Prior to carrying out any re-analysis, I thought about how this substantial change might have come about.

There are two factors which I suspect contribute. The first is the change in the UK’s age profile over the last 19 years. The ONS methodology statement summarises UK population changes between 2005 and 2023 in their Figure 3. The increase in the population of people over 70 was huge over that 19-year period (35.4 per cent). Over the period 2015 to 2023, the period that might be of most interest in calculating excess deaths, the increase in the population of people over 70 was about 16.8 per cent. This is a substantial change in the very age range within which the bulk of deaths will occur. Consequently, the data within the ONS dataset giving the age profile against month is valuable as a means of taking these changes into account.

The second factor is that on which my suspicions alight. In their previous methodology, the ONS defined the expected number of deaths in 2022 as the average of deaths registered in years 2016, 2017, 2018, 2019 and 2021. For year 2023, they defined expected deaths as the average of years 2017, 2018, 2019, 2021 and 2022. But if one wishes to examine the hypothesis that there has been an increase in excess deaths not directly attributable to covid itself (as opposed to associated interventions), then the baseline must be taken prior to the period in which the hypothesised factor applies. The ONS ‘contaminate’ their baseline with data from post-covid years and this is indisputably inappropriate for the purpose of examining said hypothesis.

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