Some time after you've purchased a stock, you probably have a pretty good idea as to whether you made a good decision or not. This is because you likely follow the stocks you have purchased fairly closely. This feedback mechanism allows you to fine-tune your stock purchase criteria so that you don't make the same mistakes again. But often, some of the best lessons to be learned come from the stocks you didn't buy, but considered buying!
Unfortunately, investors often forget about these stocks. These stocks don't make it onto their "follow" lists or spreadsheets, and have zero mind-share. As a result, they may never know if their thesis was correct. Not utilizing this potential feedback mechanism for a large number of stocks can prevent one from becoming a better investor.
Consider cataloging the stocks you were close to buying, but didn't. Check back to see if the reason you didn't buy came true. Note that for individual stocks, the result might be misleading. For example, a risk you foresaw may not have come to fruition, but may have been a legitimate reason for not buying. Looking at this "non-portfolio" in the aggregate and over several years, however, should help you improve your decision criteria.