Estimating Project Activity Durations

Have you ever worked on a project where a critical resource was not available when you needed it? Has a project activity been shorter or longer than you expected? How do project managers plan resources for these activities?

Activity duration estimates are an important input to project resource planning. Activity duration estimates are quantitative assessments of the likely number of work periods required to complete an activity. The estimates should include the range of possible results. Project managers use activity duration estimates as a basis for scheduling time and resources for a project.

The project manager uses the WBS to determine which activities are involved in completing the project. Then using the tools and techniques of activity duration estimating, he prepares estimates for activity duration.

The estimates include an "optimistic," "most likely," and "pessimistic estimate," listed in that order. For example, activity 1 could take 10 days to complete, will most likely take 12 days, but could take as long as 16 days.

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. In the example given earlier, the duration estimate would be two weeks, plus or minus two days, to indicate that the activity will take between 12 and 16 days.

Project managers also calculate the probability of the estimates being correct. For example, a project manager might estimate that there is a 15 percent probability that an activity duration will exceed 3 weeks, and an 85 percent probability that it will be less than 3 weeks.

John, a project manager for Quick-as-a-Wink computer consultants, has determined the activities involved in a software development project. One of those activities is scripting the content. John determines that the scripting activity should take 240 working hours to complete. He will use these hours to calculate his activity duration estimates.

John's activity duration estimates include the duration and the probability of the estimate being correct. John estimates that the scripting activity will take six weeks ± three days. There is a 75 percent probability that the scripting activity will take more than six weeks, and a 25 percent probability that it will take less than six weeks.

Activity duration estimates form the backbone of the project schedule. A properly prepared schedule will lead to better resource planning. And better resource planning will lead to a more successful project.