Dear Mr Rennie,
Thank you for your letter of 21 January about the First Minister’s use of statistics from the Coronavirus Infection Survey (CIS).
CIS estimated that in the week ending 15 January 2022, 5.47 per cent of people in England and 4.49 per cent of people in Scotland had COVID-19[1]. The First Minister was comparing these two proportions and correctly stated that the figure for England was more than 20 per cent higher than the figure for Scotland. It would also be correct to say that the prevalence of COVID-19 was around one percentage point higher in England than in Scotland. Quantitative comparisons between the two estimates should take account of the precision with which they are available[2], but the data does suggest that the rate of infection is lower in Scotland than in England.
The distinction between percentages (parts per hundred) and percentage points (the simple difference between two percentages) can be made easier to understand by quoting the two numbers being compared. For clarity, when publishing results from CIS the Office for National Statistics gives the absolute number of people with COVID-19; the percentage of the population with COVID-19; and the number of people with COVID-19 as a ratio to the whole population (for example, “1 in 20 people”).
Yours sincerely,
Sir David Norgrove
[1] Coronavirus (COVID-19) Infection Survey, UK: 21 January 2022. Headline results from this bulletin were published on 19 January.
[2] The 95% credible intervals were between 5.29% and 5.64% in England and between 4.03% and 5.00% in Scotland. Credible intervals give an indication of the degree of uncertainty of the estimates. More information on credible intervals can be found in the CIS methods and further information article.
Related links:
Sir David Norgrove response to Jackie Baillie – COVID infections in England and Scotland