What is PERT and how does it differ from CPM?

PERT (Program Evaluation and Review Technique) uses probabilistic activity durations: three estimates (optimistic, most likely, pessimistic) combined to calculate expected duration and variance. Expected = (O + 4M + P)/6, Variance = ((P-O)/6)^2. Project duration variance is sum of critical path activity variances (assumes independence). Probability of meeting deadline calculated using normal distribution. CPM uses deterministic (single) durations. PERT useful for R&D or unique projects with duration uncertainty; CPM for repetitive construction with reliable estimates.