> This sounds to me like a distinction without a difference. In both cases, the net result would be approximately 0 effect on the overall mental health of the p

> This sounds to me like a distinction without a difference. In both cases, the net result would be approximately 0 effect on the overall mental health of the population being studied. I believe OP is saying that the underlying assumption here is that students do not get happier during their time during university without lockdown. If for instance 2/3 students left university happier than when they started pre-lockdown/covid, this would need to be accounted for in any study to make it meaningful.