Consideration of PERT assumptions

After a very brief exposition of the PERT method, a critical discussion is given of three basic assumptions of PERT. The assumptions are: 1) that a β distribution defined between the pessimistic and optimistic time estimates adequately represents the time required to perform an activity; 2) that the standard deviation of that distribution equals 1/6 the range; and 3) that the probability of completing a project by a given date can be computed by calculating the critical path through the network and invoking the Central Limit Theorem. The significance and correctness of each of the assumptions are considered and suggestions are made for modifying them. The effects of these suggestions are shown on a typical PERT network. Other features of the networks which could simplify PERT calculations are mentioned.