The Three Time Estimate PERT Approach

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You won't find the three time estimate approach to be in great demand. After all, if we have such a terrible time arriving at a reasonable single time estimate, won't the PERT approach just give us a very precise error? This is certainly possible, and we have to evaluate the justification for either estimating mode on a case basis. Let's look at some of the advantages and disadvantages of each mode.

First of all, project managers seem to agree that the most common weakness of project schedules is the task estimates. We have trouble estimating the duration of tasks, as well as the effort required to execute the tasks. There are volumes of writings on the problems of task estimating, and there would be considerably more published on the subject, if anyone had any really good solutions for the problem of estimating. Given the softness in our base estimates, what do we gain from the triple estimate approach?

First, we are more likely to gain precision in the time estimates. When we ask a performer to estimate the duration of the task, we often get a biased estimate. The performer may be overly optimistic, assuming that everything will go well (Murphy is on someone else's job). Or the performer may be afraid to make a commitment based on a best guess, so he adds a little time as a safety factor. So just what does 10 days mean? Is it 10 days if everything goes well, but more likely to be 13 days? Or is it most likely to be 8 days, but we'll add a couple of days as a cushion? With the PERT approach, we can ask for three distinct time estimates. An optimistic estimate is usually a duration that would be achievable about 10 percent of the time.* Likewise the pessimistic estimate is usually a duration that would occur

"Note: Early PERT practices used 1% and 5% probability for the extremes. Modern practice has left these values up to the user. Other options are to select a standard deviation formula and a distribution curve, rather than three distinct time extimates.

about 10 percent of the time. The third estimate is the most likely, which we are now able to obtain without deliberate bias. The traditional PERT formula, for calculating task durations, is (A + 4B + C) / 6, where A is the pessimistic, B is the most likely, and C is the optimistic (the most likely is given a weight of four times the optimistic and pessimistic). But there are other options, as we will soon see.

Other advantages are: (1) we gain a range of task and project durations, (2) we can adjust weight factors to generate schedules with higher or lower confidence factors, and (3) we can evaluate the potential for achieving any selected project end date. We also expand the capability for performing what-if analyses. We can use this increased information about durations in our analyses of the schedule, whether performed by simple observation or via computerized probability analysis.

It's time to look at our three computer-based approaches to duration analysis. In Chapter 6.2, we argue that traditional CPM schedules produce a project end date calculation that could have a 50 percent or less possibility of being met. These programs address this issue, in varying degrees of sophistication. They are easy to understand, even if you are sigmaphobic. But ease-of-learning and ease-of-use do increase with the level of sophistication. I don't necessarily recommend them for everyone or every project. But when meeting a schedule date is important, and especially when there are dire consequences for missing schedule deadlines, these three programs will generate better estimates and an understanding of the potential (or improved confidence) for achieving the end dates.

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Responses

  • sinit semhar
    What are the tgree kinds of time estimates in pERT?
    3 years ago
  • Doris
    How to develop develop the three time estimates a PERT approach would require?
    1 year ago
  • kristy
    What are the three time estimates related to PERT?
    9 months ago

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