What we found was sobering. Our analysis suggested that managers are indeed highly prone to believing that their projects are one of a kind even though few, if any, actually are. This causes them to think they have nothing to learn from other projects. Most important, it leads them to underestimate risk and overestimate opportunity and thus make poor decisions. Specifically, the more distinctive managers consider a project to be, the more likely it is to exceed its budget and the more likely the overrun is to be considerable. That led us to the conclusion that improving project performance has less to do with managing the activities involved and more to do with addressing how project managers make decisions.