This paper reconsiders the PERT scheduling problem when information about task duration is incomplete. We model uncertainty on task durations by intervals. With this problem formulation, our goal is to assert possible and necessary criticality of the different tasks and to compute their possible earliest starting dates, latest starting dates and floats. This paper puts together various results and provides a complete solution to the problem. We present the complexity results of all considered subproblems and efficient algorithms to solve them.