In reply to Ke Tang.

Please re-read the first six paragraphs. If an expert gives you an estimate of the minimum time (a), the most likely time (b), and the maximum time (c), you can use the previous formulas to compute the parameters (alpha and beta) for a probability distribution that models the time as a random variable. The probability distribution is called the beta distribution. The (alpha, beta) values are the ones that make the beta distribution have a minimum at 'a', a mode at 'b', and a maximum at 'c'.