PERT

Duration estimation ala PERT

Program Evaluation and Review Technique (PERT) originated in the 1950s during the U.S.N. Nautilus nuclear submarine project. PERT has terminology and concepts all its own:

•    Optimistic = O
•    Most likely = ML
•    Pessimistic = P
•    Task variance
•    Normal distribution
•    Task duration as a continuum of possibilities (probability)
•    PERT equation = [(O + 4 x ML + P) / 6]

The result of the PERT equation is a weighted average that attempts to represent the conflation of the three varieties of estimate.  It is possible to down play the most likely estimate by reducing the multiplier for any given component; for example, software development duration is often optimistic among all team members. Additionally, the approach assumes that all components of model are sensible estimates themselves.