When you create project plans, you need to assess the time required to complete individual activities. Use duration estimates to get an idea of this time and determine the schedule you need to adhere to in order to complete projects. You can use various methods to estimate activity duration, depending on the nature of the activities.

PERT Method

The Program Evaluation and Review Technique, or PERT, estimation method is a weighted average of three scenarios. First, compute the averages for optimistic, pessimistic and most likely time scenarios. PERT gives more weight to the most likely scenario, so multiply that average by four. Combine this result with the optimistic and pessimistic averages, and divide the result by six to come up with a final estimate. The optimistic scenario presumes all goes well, and you can complete the activity without issues. Conversely, with the pessimistic scenario, you imagine everything will go wrong and it’ll take longer to complete the activity. The most likely scenario assumes you can complete the activity without surprises. The PERT estimation method is a good option if you’re uncertain about the activities. It also enables you to consider all associated risks.

Analogous Estimation

The analogous, or top-down, estimation relies on information from similar projects to determine the activity duration for a current project. You’ll need historical data and a degree of expertise about the similar projects, because the reliability of your estimation depends on how closely the activities match the projects you're using as comparisons. Use this method at the beginning of a project when you don’t have all the details. Adjust the estimates as you learn more about the tasks and how long they can be expected to take with the resources available.

Parametric Estimation

The parametric estimation is similar, but more accurate, than the analogous estimation. To use it, multiply the number of units you need by the time it takes to produce the units. You’ll need historical information about similar activities to complete your estimate. The method is scalable. This means if your historical data tells you that it takes one person an hour to produce one unit, you can reasonably estimate that you can complete three units within one hour if you allocate three workers to the task. When you use this method, it's important to account for all tasks that impact the activity. For example, if the workers spend part of the time preparing materials, account for that time in your estimates.

Expert Judgment

If your project is complicated and a number of factors can influence the duration of your activities, you might want to use expert judgment to estimate activity duration. Experts knowledgeable in a particular area can judge the time and resources you’ll need to complete activities in that area. Be careful when choosing experts or using their estimates, because they may have certain biases that might influence their estimates. You also can gather estimates from external experts. If you can get external estimates at reasonable cost, it might be prudent to rely on them instead of generating estimates internally.

The Delphi Technique

The Delphi technique makes use of group intelligence to determine activity duration. The technique involves gathering opinions from several experts and then sending the responses back to the experts for their review. They can change their opinions after reviewing the responses. The process might involve several rounds, as you want to investigate differences of opinion and get to a consensus. To reduce bias and prevent individual experts from overly influencing results, experts submit their opinion anonymously. You can use a third party to gather the opinions.

Work Breakdown Structure

Certain activities might be too large or complex for a reliable duration estimate. If an activity takes up more than 10 percent of the project schedule, you might want to break it into several different tasks. You can use a work breakdown structure to reduce these activities into smaller, more manageable tasks. Doing this enables you to set priorities and estimate the duration of tasks more accurately. A work breakdown structure also is useful for building accountability, because you can assign specific tasks to designated project participants.