I have a somewhat more fundamental concern that there might be a bit of the post hoc ergo propter hoc variety.

Do firms doing buybacks do well because of it? Or, for example, do firms making more cash than they can usefully invest do better than average, and those firms tend to have a limited range of option for the excess?

I once did some consulting at the HQ of Bristol Myers Squibb. This was a fantastically profitable business at the time. Incidentally not because they were particularly well managed (it would turn your hair white), but that they were in a business through which flowed oceans of cash.

Anyway, I noticed that their head office had an extraordinary art collection, everywhere you looked. I imagine many such very fat headquarters were like that. So, imagine a study that attempts to show that a great art collection causes great shareholder returns. It would be a great fit to the data, but in fact it's just synchronicity: neither item (art and shareholder returns) caused the other, but rather both were the product of a hidden third cause: tons of money rolling in the door.

Now, maybe buybacks do cause higher prices a bit at the margin. But I don't think that this type of retrospective study can demonstrate it in a meaningful way.

Jim