IMPROVED MOMENT-ESTIMATION FORMULAS USING MORE THAN 3 SUBJECTIVE FRACTILES

PERT-type subjective estimations are used in many stochastic decision models to estimate the random variables' mean and standard deviation ( s.d.). The approach is based on the beta-distribution assumption; also , mast PERT-type formulas use only three estimated fractiles. We point out that: (if it is desirable to consider a substantially richer set of distributions than the beta in developing PERT-type formulas; (ii) it may be beneficial to use more than three fractile-estimates in PERT -type formulas. We then develop formulas for estimating the mean and s .d. that are based on a substantially richer set of distributions than the beta and that use more than three estimated fractiles. These form ulas perform better than the best currently-available formulas when th e subjective distribution is not restricted to be beta.