Once the research is done, they decide on a fund for life, as if they couldn't have been mistaken in their judgment, or as if none of the factors affecting their decision will ever change again. Performance records are useful in identifying funds that perform a certain way during certain market conditions. For example, aggressive growth funds as a group tend to do well in bull markets, but you still have to own funds based on how well they do while you own them. Use performance records as a way to narrow the field. Use current performance to decide whether or not to own.
Mutual funds are like market indexes or industry groups.
They are made up of groups of stocks, and they will perform better or worse depending on how the market is treating the particular stock mix of a particular fund. There is a tremendous amount of rotation of money between industry groups, and an individual mutual fund will fare better or worse based on this rotation. However, you don't have to know any of the details of why your fund is moving up, down, or sideways. All you need to know is WHAT it is doing and to react accordingly. Mutual fund investors should endeavor to stay in mutual funds that are performing well and avoid those that are not.