http://www.csai.cn/tszl/pert.htm
Pert (plan review technology, Program Evaluation An Review Technique) is the development of more than 3,000 contractors and research institutions at the end of the 1950s, and its theoretical foundation is assumed that the project duration and the entire project completed Time is random, and obey some probabilistic distribution. Pert can estimate the probability of completing the entire project in a certain time. PERT and CPM apply very wide in the progress plan of the project, and this paper explains this technique in this paper.
First, the time estimation of the event
Pert is estimated by three different situations for the completion time of each project activity:
1. Optimistic Time - Any thing is going well, completes a certain job.
2, most likely time (Most Likey Time) - Under normal circumstances, complete a certain job.
3, pessimistic Time - the most unfavorable situation, complete a work.
Assuming three estimated obedience are distributed from beta, thereby calculating the expectations of each activity TI:
Among them: AI means the optimal time of the Item I, Mi - the most likely time of the Item I, BI indicates the pessimistic time of the Item I.
According to the variance calculation method of the beta distribution, the duration of the i-th activity is:
For example, the construction of a government OA system can be decomposed into four activities such as demand analysis, design coding, testing, installation deployment, each activity, no time overlap, and the completion time of the activity is estimated below:
Second, project cycle estimation
Pert believes that the completion time of the entire project is the sum of the completion time of each activity, and obeys a normal distribution. Whole project
Because FIG. 2 is a normal curve, according to the normal distribution law, the probability completed between 47.304 days and 54.696 days in the ± σ range is 68%; in the ± 2σ, the 43.608 days to 58.393 days The probability of completion is 95%; the probability completed in 39.912 days to 62.088 days in ± 3σ is 99%. If the customer is required to be completed within 39 days, the probability that can be completed is almost 0, that is, the project has a minimum cycle that is uncomfortable, which is an objective law. By checking the standard normal distribution table, you can get the probability that the entire project is completed at a certain time. For example, if the customer is required to complete within 60 days, the probability that the possible completion is:
Third, small knot
In fact, the construction period estimation and progress control of large projects is very complicated, often need to be used in conjunction with CPM and PERT, and obtain critical paths with CPM, and then perform the expectations and variances for each activity on critical paths, and finally The probability that is completed in a certain period of time.
Pert also tells us that any project has a minimum cycle that is irreparable. This is an objective law. Don't blindly commit users without customer views, otherwise it will definitely be punished by the objective law.
