In schedule risk analysis we do not believe that all activities will be at their most pessimistic or most optimistic durations in the same project. But, what would happen if many activities were to overrun their estimated durations together on our project? That event could cause very large overruns of the total project completion date. This is because the long durations are summed down the paths, reinforcing overruns of other long-duration activities on the same path. We can handle this common occurrence in probabilistic analysis using correlation between uncertain activity durations.