Past results can be helpful when analyzing an investment, but it's important to look at a long time horizon. Since the past success does not guarantee future performance. Say for example if a stock goes up 15% in one year, you don't know whether it is a good investment now. It doesn't tell you whether it will be good in the future. It only tells you what it did over that one-year period. Hence one should be a disciplined investor and not always look back.