Three-point method or weighted three-point estimate or PERT-score is a method for estimating time and effort in project management . It extends the two-fold method, consisting only of the best and worst-case time scenarios with the additional parameter “most likely execution time”.
Weighted three-point estimate
The use of a weighted three-point estimate is very useful if we assume that the real time is closer to the minimum or maximum estimate.
The reason for this may be, for example:
- The project is very risky, so the appearance of the best scenario is very unlikely
- The person who is working on the project is either very optimistic or, on the contrary, very pessimistic forecasts.
In these cases, an additional assessment of the probable case (Most Likely Case) is recommended in addition to the minimum (Best Case) and maximum estimates (Worst Case). The value is determined by the expression
The probable case thus gets stronger weighted. For this method, it is very important that the draft event assessment is regularly updated, as its uncertainty decreases as the project progresses.
Criticism
The weighted three-point estimate is valid only for the symmetric probability function and does not provide any information about the estimated absolute probability of occurrence of the event. Both deficiencies can be eliminated by using an explicit three-point estimation method with absolute probability.
Explicit three-point estimation method with absolute probability
Tom DeMarco, in his book Bärentango, demonstrated that estimates can be an asymmetric probability function. That is, it is more likely that pessimistic values are significantly higher than optimistic ones. At the same time, Tom DeMarco raised the idea that the absolute probability can be calculated by integrating the area under the curve.
To estimate or just to calculate a certain probability, you can use the appropriate Excel macro on the site [1] .
If several work packages are dependent on each other, as a result, the likelihood function can be calculated using the convolution operator. Corresponding calculation programs can be found on the Internet.
Literature
- Tom DeMarco, Timothy Lister: Bärentango. Hanser Fachbuchverlag, Leipzig 2003, ISBN 3-446-22333-9
Links
- Excel for probability calculations
- Using the method for writing computer programs
- It Takes Three Essentials from Visionary Tools
- Three-Point Estimate Approximations from www.super-business.net
- Risk and duration estimates: 3 point estimating from www.4pm.com