tailieunhanh - Project risk management processes techniques in sights phần 6

B có xu hướng chi phí rất nhiều so với dự kiến , bởi vì sự cần thiết để giữ cho các dự án mục tiêu, hoàn toàn ngoài từ các nguồn khác theo định hướng thị trường phụ thuộc. Đặt một mô hình cách khác, chi phí và thời gian của sự không chắc chắn điều đó không rõ ràng xem xét | Provide more probability distribution detail 187 Expected value . M Ư 3 Figure The triangular distribution a component at a particular time given survival to that point in time. The analyst specifies parameters that specify the timing of the burn-in steady-state and wear-out periods together with failures rates for each period. The software then produces appropriate bathtub and failure density curves. Woodhouse 1993 gives a large number of examples in the context of maintenance and reliability of industrial equipment. A popular choice for many situations is the triangular distribution. This distribution is simple to specify covers a finite range with values in the middle of the range more likely than values of the extremes and can also show a degree of skewness if appropriate. As shown in Figure this distribution can be specified completely by just three values the most likely value an upper bound or maximum value and the lower bound or minimum value. Alternatively assessors can provide optimistic and pessimistic estimates in place of maximum and minimum possible values where there is an x chance of exceeding the optimistic value and a 100 x chance of exceeding the pessimistic value. A suitable value for x to reflect the given situation is usually 10 5 or 1 . In certain contexts estimation of a triangular distribution may be further simplified by assuming a particular degree of skewness. For example in the case of activity durations in a project-planning network Williams 1992 and Golenko-Ginzburg 1988 have suggested that durations tend to have a 1 2 skew with the most likely value being one-third along the range . 2 M - L U - M in Figure . The triangular distribution is often thought to be a convenient choice of distribution for cost and duration of many activities where the underlying processes are obscure or complex. Alternative theoretical distributions such as the Beta Gamma and Berny Berny 1989 distributions can be used to model more rounded