Three-point estimation

Called The Three Times- way and 3- point estimate or PERT estimation, is a method for the estimation of time or effort in project management. It extends the two-time method by an additional weighting of the estimate of the best-case, Likely -case and worst-case.

Weighted 3-point estimate

The application of the weighted 3-point estimate is useful if it can be assumed that the actual time is closer to the minimum or maximum estimate.

The reasons for this can be for example:

• The project is extremely risky, so that the occurrence of the best-case is very unlikely.
• The underestimated employee experience has shown very pessimistic or optimistic.

In these cases, the additional estimation of the most likely case ( Likely Case ) is recommended in addition to the minimum estimate ( best-case ) and maximum estimate ( worst-case ). The plan selection via the formula

Be calculated. The probable case is obtained in this way a strong weighting. With this method, however, it is essential that the project manager updates the estimates regularly as the uncertainty decreases during the project (see Cone of Uncertainty )

Criticism

The weighted three -point estimate is only valid for a symmetric probability function and provides no information on the expected absolute Eintreffwahrscheinlichkeit the estimate. Both drawbacks are remedied by the explicit 3-point estimate an absolute probability.

Explicit 3-point estimate with absolute probability

Tom DeMarco has shown in his book bears Tango for the first time that estimates are subject to an asymmetric likelihood function. That is, the probability that a pessimistic values ​​occur, is significantly higher than in the optimistic value. At the same time Tom DeMarco has raised the idea that the absolute probability can be calculated by integrating the area under the curve.

If one approximates the probability function by a simple straight line, can the absolute probability for a given estimate or the estimate to a defined probability calculated easily. An appropriate Excel macro for calculation finds among themselves.

If several work packages are dependent on each other, the resulting likelihood function can be calculated with the convolution operator. An appropriate Windows program for calculation finds among themselves.