In a wide range of social and economic situations, people are rewarded on the basis of an assessment of their past performance, but often an objective account of such performances is not available. As result, the assessors are forced to rely on their memories of past events.
People usually hope that successes will be remembered and failures forgotten, and they assume that the more recently an event has passed, the fresher it will be in the assessors mind. However, experiments have shown that memory is more complicated and more interesting than you may think.
Our memories actually operate on the principles of similarity and repetition. Similarity refers to the phenomenon wherein current events trigger memories of similar past events, while repetition refers to the fact that recalling the memory of an event increases the likelihood that it will be remembered in the future. People also have the tendency to recall events incorrectly or to "remember" events that never even occurred.