Activity duration estimating is the process of taking information on project scope and resources and then developing durations for input to schedules. The inputs for estimates of duration typically originate from the person or group on the project team who is most familiar with the nature of a specific activity. The estimate is often progressively elaborated, and the process considers the quality and availability of the input data. Thus, the estimate can be assumed to be progressively more accurate and of known quality. The person or group on the project team who is most familiar with the nature of a specific activity should make, or at least approve, the estimate.
This technique is also known as PERT (project evaluation and review technique) estimates.
If only one estimate were to be used for a task, then there is a high probability that the estimate will turn out to be slightly higher or lower than reality. But suppose when seeking an estimate, that you asked for three estimates instead of just one.
The approach here is to ask for a pessimistic or worst-case estimate, a realistic almost likely case estimate, and an optimistic or best-case estimate. You may wish to use work hours in carrying out this technique and then convert them to duration. Alternatively, you may use this technique directly with durations such as days:
The three estimates stated above are replied in a formula that produces a weighted average of the three and results in a single more realistic estimate for the task duration.
Consider this example. You ask a software developer how long a program will take to write, most likely you will get a number that is pretty optimistic. To get a realistic estimate you would ask him to think of worst-case scenario (pessimistic estimate, tP), best-case scenario (optimistic estimate, tO), most-likely scenario (tM), and estimate durations for each of them. Then you run the following formula to calculate a number that is closer to reality.
Expected duration = [Optimistic duration + (4 x Most likely duration) + Pessimistic duration] / 6
tE = [tO + 4tM + tP] / 6
Another formula to remember is that of standard deviation for each estimate is that standard deviation = (pessimistic – optimistic) / 6
The approach here should be to consider any risks that may occur when carrying out a particular activity and the impact such risks would have all the activity completion. Examples of such risks might be rework, errors, technical difficulty, resource availability, etc. Based on such risk analysis, a contingent amount would be added to the activity. For example, if reworked was a risk and likely outcome, and it was determined than an extra 5 hours work would result, then either more resources would be added to the activity, or its duration increased by an extra day.