|Its called "other factors". |
Company XYZ announces a buyback. Then the next quarter a new business opportunity comes up, the quarter after that they start realizing revenue and within the year they are much more profitable and their stock price goes up. Does that somehow prove the stock buyback was a brilliant move? Of course not.
Company ABC announces a buyback. A couple of quarters later North Korea nukes someone and the stock market goes in to the toilet along with ABC's stock. Does that somehow prove that the stock buyback was idiotic? Again No.
What you need to know is not what happens to the stock but what would have happened without the buyback. Just considering the later change in the stock market as the only way to score how good of decision that buyback was only makes sense if the buyback is the only major influence on the stock price in the period your examining,