Three times method

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The three-time method , also known as 3-point estimation or PERT estimation , is a method for estimating time or effort in project management . It extends the two-time method by adding additional weighting to the estimate of the best-case, likely-case and worst-case.

Weighted 3-point estimate

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

The reasons for this can be, for example:

• The project is extremely risky , so the occurrence of the best case is very unlikely.
• Experience shows that the appraising employee is very pessimistic or optimistic.

In these cases, it is recommended to estimate the most likely case (Likely case) in addition to the minimum estimate (best case) and maximum estimate (worst case). The plan value can be determined using the formula

${\displaystyle {\frac {{\text{BestCase}}+4\times {\text{LikelyCase}}+{\text{WorstCase}}}{6}}}$

can be calculated. In this way, the probable case is given a heavy weight. Even with this method, however, it is essential that the project manager updates the estimates regularly, as the uncertainty decreases in the course of the project (see Cone of Uncertainty )

criticism

The weighted 3-point estimate only applies to a symmetrical probability function and does not provide any information about the expected absolute probability of occurrence of the estimate. Both disadvantages are eliminated by the explicit 3-point estimation with absolute probability .

Explicit 3-point estimate with absolute probability

typical probability function of an estimate as the basis for the explicit 3-point estimate

Tom DeMarco showed for the first time in his book Bear Tango that estimates are subject to an asymmetrical probability function . This means that the probability that pessimistic values ​​will occur is significantly higher than with optimistic values. At the same time, Tom DeMarco came up with the idea that the absolute probability can be calculated by integrating the area under the curve.

If the probability function is approximated by simple straight lines, the absolute probability for a given estimate or the estimate for a defined probability can easily be calculated. If several work packages are dependent on each other, the resulting probability function can be calculated with the convolution operator . Macros or programs for both can be found on the Internet.

literature

• Tom DeMarco, Timothy Lister: Bear Tango. Hanser Fachbuchverlag, Leipzig 2003, ISBN 3446223339

Web links

• 3-point estimate with probability. In: Speed4Projects . 
• Chaining estimates. In: Speed4Projects . 

Individual evidence

1. ↑ 3-point estimate with probability. In: Speed4Projects . Retrieved March 7, 2019 (Excel for calculating the exact probabilities). 
2. ↑ Chaining estimates. In: Speed4Projects . Retrieved March 7, 2019 (software for calculating when 3-point estimates are chained).