Three point estimates a short cut method

If there is insufficient data to describe a continuous probability distribution for a variable (as with the area of a field in an earlier example), we may be able to make a subjective estimate of high, medium and low values. If those are chosen using the p85, p50, pi 5 cumulative probabilities described in Section 6.2.2, then the implication is that the three values are equally likely, and therefore each has a probability of occurrence of 1/3. Note that the low and high values are not the minimum and maximum values. 

To estimate the product of the two variables below, a short cut method is to multiply the low, medium and high values in a matrix (in which numbers have been selected). 

Note that the low value of the combination Is not the absolute minimum (which would be 4, and is still a possible outcome), just as the high value is not the maximum. The three values (which are calculated by taking the mean of the three lowest values In the matrix etc.) represent equally likely outcomes of the product A B, each with a probability of occurrence of 1/3. 

This short-cut method could be repeated to include another variable, and could therefore be an alternative to the previous two methods introduced. This method can always be used as a last resort, but beware that the range of uncertainty narrows each time the process is repeated because the tails of the Input variables are always neglected. This can lead to a false impression of the range of uncertainty in the final result. 

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