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How to estimate project completion times

To determine the duration of an individual task or calculate when a project will complete, Lawrence Oliva suggests using your expert judgment, top-down estimating, and parametric formulas such as PERT.

Lawrence Oliva, PMP
Lawrence Oliva, PMP

"How long will this take?" and "When will the project be finished?" are two of the most asked questions project managers must answer accurately. Experienced PM's have crisp responses such as "two weeks," "three days," or "November 7, 2008." Those types of responses are what senior managers and clients expect and base their perceptions of PM competence on.

Being able to calculate how long activities take is (and should be) a core PM competence. Of course, unexpected events occur that are out of the control of the PM, such as weather disasters, and events such as the 9/11 attacks. Any situation of that magnitude will have a massive schedule impact, but fortunately those are rare occurrences.

To determine the duration of an individual task or calculate when a project will complete, a PM can apply one of several proven estimation methods, including the following:

  • Expert judgment: Should be used when guided by historical information

  • Top-down estimating: Used when detail information is limited and is most reliable when previous activities are similar and the individuals making estimates have the needed expertise

  • Parametric formulas: One example is PERT

The first two methods substantially rely on the PM's (or another person's) experience and subject matter knowledge of the situation. When performed by an experience person, they can provide somewhat better information to management than "sometime in the future."

When working with inter-dependent project tasks, I often use the last method (parametric formulas) to confirm my estimates developed using the first two methods. If all three provide similar schedule estimates (they will not offer precisely matching answers) I know my approach and projections are probably accurate. What is PERT? It stands for Program Evaluation and Review Technique. There are several elements to PERT, one of which is a formula that combines the optimistic time, the pessimistic time, and the most like time into a weighted average. The formula is:

Expected Time = (optimistic + 4X most likely + pessimistic) / 6

The PM or project scheduler estimates the optimistic, most likely, and pessimistic durations and plugs them into the formula. For example, optimistic duration = 10 days, most likely duration = 12 days, and pessimistic duration = 17 days.

Expected time = (10+(4x12)+17)/6 or 12.5 days

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The PERT formula (in this example, 12.5 days) becomes the starting point for further adjustment such as adding more time for contingency purposes or reducing time due to acquiring additional resources.

How else can a PM deliver tasks or projects on schedule? By decomposing tasks into small work packages that can have their progress measured by the day, week, or month. Being alerted that task delays have occurred within a day or two of when they are supposed to be completed (which is possible when tasks durations are short) greatly assists the PM in developing corrective action plans required for task schedule recovery.

Yes, project estimates are sometimes guesses. Being able to make an educated, consistent estimate using one of these methods, combined with your common sense, will set you apart from your peers.

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About the author: Lawrence Oliva is a senior consultant and program manager with CH2M HILL, a global engineering and program management company. Based in the Washington D.C., area, he currently leads mid- to large-size IT programs for federal government and commercial clients that have unique technical challenges that require risk management, earned value analysis, cost/schedule forensics, biometric security, and "Green IT" implementation experience.


This was last published in September 2008

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