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The Effect of Risk Analysis on Start Dates

In general, the early start dates calculated by risk analysis will be later than those in a project using standard time analysis.

This is because in simulating a project with parallel paths of activities, it is always the latest of the early start dates that controls the scheduling of the remainder of the project.

This tendency of the latest early dates to control the subsequent early dates also tends to skew the shape of the probability distribution of all early start dates to the "right." This skewness undercuts the reliability of using the calculated mean date and standard deviation to estimate confidence intervals. In many cases, for example, it may turn out that adding three standard deviations to the "right" of the mean early start results in a value outside the range of the curve, thus rendering confidence levels based on standard deviations inaccurate.

While many users are not concerned with late start dates, you may want to use this information to avoid starting expensive activities with lots of float much earlier than absolutely necessary by assigning that activity a target start date. Assuming that you want these target start dates to pose a minimal risk for delaying the project completion date, you would want to select a date at a very low confidence level (for example 1% to 5%). Selecting a later start date (in other words, a date with a higher probability of actually occurring) increases the likelihood of causing further delays in the completion of the project.

Placing an effective target start date on an activity alters the early date's distribution curve and increases the probability that the activity will be critical and, thus, affect subsequent activities. This effect can be minimized by limiting the number of activities with target start dates and by specifying dates with very low probability.

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