Project Outputs: Activity Duration Estimates

As a project manager, you can help ensure the success of your project by recognizing and understanding the various outputs of activity duration estimating. One of those outputs are the activity duration estimates themselves.

Activity duration estimates are quantitative assessments of the likely number of work periods (usually days) required to complete an activity. Project managers use activity duration estimates as a basis for scheduling time and resources for a project.

Activity duration estimates are derived from the Work Breakdown Structure (WBS). Use the WBS to determine which activities are involved in completing the project. Then, using the tools and techniques of activity duration estimating, prepare estimates for activity duration.

The estimates should include optimistic, most likely, and pessimistic estimates, listed in that order. For example, an activity that could be completed in as few as 10 days (an optimistic estimate), will most likely take 12 days to complete (the most likely estimate), but could take as long as 16 days (a pessimistic estimate).

Only the optimistic and pessimistic estimates are used by project managers to create a range of possible results. These results become your activity duration estimates, or valid durations. Valid durations are estimates arrived at using the tools and techniques of activity duration estimating.

You also should calculate the probability of the estimates being correct. Since you're working with estimates and not actual outcomes, you won't be able to use formulas for calculating probability. Instead, you should use other techniques, such as expert judgment or analogous estimating.

For example, using expert judgment, a project manager estimated that there is a 15 percent probability of an activity exceeding three weeks and an 85 percent probability of it taking less than three weeks.

Activity duration estimates form the backbone of the project schedule. Properly prepared estimates will lead to a better schedule—and a better schedule will help lead to a successful project.