@AdamO neither is right or wrong they're just calculating different things. For instance if sample before the drug is $5,5,1,1$ and sample after the drug is $15,10,3,1$, the first method would give you a $+125\%$ average change (the average of $200\%, 100\%, 200\%, 0\%$ while the second one would say that the average has changed by $141,67\%$ (the average has changed from $3$ to $7.25$)

@FrankHarrell Sure! That's why you should fit a few times with different splits!

Start by getting a good grasp of derivation and integration. Maybe not becoming great at solving integrals, but rather trying to understand them conceptually and what they means in the context of statistics.

Stock returns are an almost perfect example of regression towards the mean. So are most games where luck plays a significant role

Note that "traditional" statistics used to prefer linear methods because the computing power just wasn't there