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Estimating Project Duration

The estimated project duration is based on the sum of the activities. With some math magic, we can predict the best, worst, and most likely scenario for how long each activity will take, and ultimately how long before we put this beast to bed and move on to the next.

To create an estimate that's accurate or close to accurate, it's really more than just adding up the activity durations. We also have to consider several factors that could wreck the schedule, drive managers nuts, and cause our hair to fall out (see my photo):

  • Constraints. A constraint is anything that restricts project options. You've hired a consultant who is only available on December 29. You've enrolled your project team in a training class on January 20. Sam the systems engineer is taking a four-week vacation in February. All of these are constraints—and there are tons more.
  • Assumptions. We all know what happens when we make an assumption (think ass and umption). For example, we assumed that the vendor was honest when saying that the new servers would be delivered by October 1. We assumed that the client was using Windows 95 or better, not OS/2. We assumed that we could have access to the job site 24/7, not just during business hours. If we don't document and share these assumptions, it's trouble.
  • Available resources. Have you ever calculated that you'd need four network engineers to pull and install the cable, only to discover that you only have two network engineers for your project? If your required resources aren't available, your project will be hurting.
  • Law of Diminishing Returns. The Law of Diminishing Returns controls yield against the amount of labor available. Suppose we have an activity that will take 40 hours with two network engineers assigned. If we add two more network engineers to the activity, can we finish the work in 20 hours? Maybe. If we add 40 network engineers to the activity, can we get done in a few minutes? Not likely. In addition, not all activities are effort-driven; many are of fixed duration. This is a nice way of saying that it doesn't matter how many experts you throw at the activity—it will still take the same amount of time to complete.
  • Parkinson's Law. Parkinson's Law states that work will expand to fill the amount of time allotted. Imagine that Joe says an activity will take 40 hours to complete, although he knows he could finish the work in just twelve hours. He's added padding to account for potential mistakes, issues, and games of solitaire he may encounter. Magically the task will grow to take the allotted 40 hours. Think of your workload the day before you leave for vacation. You can crank and get it done. But the day after vacation? Takes all day to send one email. (Maybe not you, but probably that guy in the cube next to you.)
  • Risks. Business risk can have an upside or a downside—although we usually think of risks with the downside. Most risks that come into fruition have potential to delay the project work, add activities, and in some instances require us to wave the white flag of surrender. (I'll cover risks in more detail in another article. I know, another time management issue for me. Thanks.)
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