I have data from 70 studies reporting number of observed events in two similar time periods, one before Covid and one during the Covid pandemic. The duration of the timeperiods and the size of the population were the events occurred varies between the reports but are corresponding for the paired data within each study. There is no information about the size of the population so cannot use incidence rates.

The hypothesis is that the number of events would be the same in these periods. How can I do the analysis? The problem is the denominator. Can I use the total number of events and estimate the RiskRatio as (percovid numbers/total numbers)/(precovid numbers/total numbers) ? Or simply use the Odds (percovid numbers)/(precovid numbers)? How can I estimate the CI for the estimates?

And how can I do it in Stata?