Disadvantages of the PERT Formula

Written by: Ronda Roberts • Edited by: Marlene Gundlach
Updated Aug 14, 2009

With every method in project management, there are benefits and disadvantages. This article explores the disadvantages of relying on the PERT Formula for duration and cost estimates.

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In part three of the PERT Formula series, I discussed the benefits of using the PERT Formula in project management. In this article, I will take a look at the other side of the PERT Formula: The disadvantages to using it. While the PERT Formula in many cases is a very helpful tool, there are limitations to the formula.

1. Where do the Initial Duration Estimates Come From?

One of the most criticized factors in using the PERT Formula is the question of whether the initial optimistic, most likely, and pessimistic durations of activities are truly objective. If the estimates are subjective, then it compromises the purpose of the formula. The weighted estimate and standard deviation will not accurately depict the amount of time required for each task.

2. The PERT Formula Requires Too Much Work

It takes a lot of work to come up with the numbers used in the PERT Formula - especially if the project manager is calculating the estimated duration for each work package on an individual basis. Because of this, the PERT Formula might be said to be an inefficient method of estimating the duration, start, and completion times for the project. The argument that follows is that if the PERT Formula is inefficient it should not be used.

3. The PERT Formula Isn't Practical

This argument arises out of the second potential disadvantage of the PERT formula. Because the formula requires at least three durations to be conceived of for each task, and because it requires a mathematical skill, the argument is that the formula just isn't used in practice.

4. Actual Completion Time is Under-Estimated

One of the biggest disadvantages of the PERT Formula is often times duration is under-estimated rather than accurately estimated or over-estimated. Under-estimation of time can cause huge problems in project management. Not only can it cause the project to fall behind, but it can also cause overages in budget when employees are either forced to pull overtime to meet project deadlines or the project might over extend what was budgeted resource-wise, causing a problem with over-allocation.

Can the Disadvantages be Overcome?

In short, yes. By being meticulous with the initial estimates, by estimating only in risk scenarios, and by double checking estimates with seasoned task members, these disadvantages can be thwarted. It requires patience and persistence, but the formula has been around long enough to merit that its potential benefits override its potential deficits. For additional uses of the PERT Formula, read Deanna's article "Project 2007: Estimating Task Durations Using the PERT Formula."

Ronda Roberts Aug 23, 2011 7:19 PM

RE: Disadvantages of the PERT Formula

There are two sides to everything - in part three of this series, I discuss reasons why one might want to use the Pert Formula for estimating and the benefits to it.

DE Aug 21, 2011 11:09 PM

Supposed flaws with the Beta formula

With all due respect to Ms. Roberts, I disagree completely with what she calls flaws.
1. Where do the initial estimates come from? Answer: The same place they would come from if you used a point estimate instead of factoring in the best and worst case. How can factoring them in hurt?

2. The PERT formula takes too much work. Sometimes. Working for a client, sometimes I don't have constant access and can't use Beta for everything. So? Use it when you can. That's not a flaw.

3. The formula requires a mathematical skill? If you can't figure out this formula, you shouldn't be managing a trip to the bathroom by yourself, let alone a real project.

4. Beta leads to underestimation? How? Most people would simply estimate a task duration as what happens typically. Beta factors in the worst case so leads to a more conservative estimate.

The Beta formula is not perfect. No estimating technique is. But it's better than a simple point estimate.