> Is this not a randomised control trial? No. I am not quite sure how one might even think the words apply here. If it were, participants would be randomly as

> Is this not a randomised control trial? No. I am not quite sure how one might even think the words apply here. If it were, participants would be randomly assigned to the the "control" group where they do not get Covid, but would be told they had Covid (to control for the negative placebo of being told they are now afflicted with a horrible long term malady) and the "treatment" group which would actually be given a Covid infection. Assuming such a study were doable, one can then use a "within participant" comparison which is much more powerful than comparing statistics across groups. >> We sought to confirm whether there was an association between cross-sectional cognitive performance data from 81,337 participants who between January and December 2020 undertook a clinically validated web-optimized assessment as part of the Great British Intelligence Test, and questionnaire items capturing self-report of suspected and confirmed COVID-19 infection and respiratory symptoms. There is no randomized assignment here. Susceptibility to report a Covid infection without positive proof is a confounding variable. I suspect that susceptibility is correlated with other cognitive issues. A "within participant" comparison as the GP brought up would avoid these kinds of issues.