The true mortality rates for UK people born between 1919 and 1929 are understated in official figures produced by the Office for National Statistics (ONS) due to the impact of uneven birth rates, academics say.

A joint study from the Pensions Institute — part of Cass Business School, City University London – and Heriot-Watt University, Durham University Business School and Prudential Financial, found that anomalies in mortality rates can often be linked to uneven patterns of birth.

The study showed these uneven birth patterns can lead to errors of more than 9% in the estimated size of some England and Wales birth cohorts.

David Blake, director of the Pensions Institute, said:  “Mortality rates are determined by the number of people who die at a given age divided by the population who remain alive at that age. 

“We have accurate data on the number who die each year, but the exposed population has to be estimated and it is usually estimated at mid-year,” he said.

The researchers found that an uneven pattern of births within a given calendar year was a major cause of error in the estimated mid-year population, Blake said.

An analysis of mortality data produced by the ONS showed a “puzzling” pattern of mortality improvements among people who were now aged over 90, going back to 1992, the institute said.

It found this was due to a combination of effects.

In 1919, births were much lower at the mid-point of the year — when they are measured — than they were on average that year, and then in 2001, a change in the method used to derive mid-year population estimates led to the number of people born in 1919 being overstated, it said.

In its conclusions, the Pensions Institute called for a “fundamental review of all official mortality data and how users interpret these data.”

Andrew Cairns of Heriot-Watt University said the study was a “reminder that real-world datasets are rarely as accurate as we would like them to be.”