tailieunhanh - Project risk management processes techniques in sights phần 7

Dự phòng lập kế hoạch có xu hướng để ước tính sự không chắc chắn thời gian sai lầm (thường là lạc quan) và không cấu trúc hoặc giải thích nó và có xu hướng hết sức để đánh giá thấp chi phí trực tiếp không chắc chắn. Xem xét tác động của kế hoạch dự phòng sẽ làm rõ sự không chắc chắn thời gian rõ ràng và làm tăng sự không chắc chắn rõ ràng chi phí trực tiếp. | 228 Evaluate overall implications confidence band . This result suggests an extraordinary degree of certainty about the sum of our 1 000 cost items The absurdity of this result should reinforce Example and the importance of dependence. More generally it illustrates the way independence induces a square root rule reduction in variability as n increases. The standard deviation of the cost of one of our cost items square root of is about 14 of but 44 is about of 10 200 a reduction by a factor of about 32 equal to the square root of 1 000 . More generally compared with a single item n 1 n 4 increases total variability by a factor of 2 n 16 increases total variability by a factor of 4 n 64 increases total variability by a factor of 8 and so on. Mean variance approaches need not assume independence and an expert system-driven probability evaluation package could usefully embrace momentbased approaches as well as CIM and Monte Carlo. Moment-based approaches offer speed and precision when appropriate assumptions hold and catastrophic systematic errors bias when appropriate assumptions do not hold. To summarize this section three types of procedure can be considered each with their advantages and disadvantages. Current industry standard software based on Monte Carlo simulation is effective if used by experts with adequate understanding of dependence in a manner reflecting the usage of CIM approaches but it is not a simple efficient solution to all evaluate phase issues. It needs to be used with care and better software is feasible even for the simple treatment of dependence discussed in this chapter. The discrete probability approach used in this book is useful at a conceptual level even if it is replaced by a Monte Carlo simulation approach for operational purposes. Mean variance approaches are also conceptually useful but need great care when used in practice especially if they tempt people to assume independence that does not exist. Conclusion In