جمعه 14 اردیبهشت‌ماه سال 1386

Chapter 4-2:Activity Duration Estimates

Activity Duration Estimates

We have defined our activities, identified all the dependencies between tasks, and developed a network diagram to depict the flow of the project work. We must be ready to complete the project schedule, right? Not quite yet—we still need a very critical component: how long each task will take to complete. Activity duration is the process of estimating the time to complete each item on the activity list. The most common measurements used to define duration are days or weeks, but this can vary based on the size of the project.

Before we explain the techniques you can use to complete your duration estimates, let’s make sure we have a common understanding as to what is meant by activity duration.

Defining Duration

When you are estimating duration, you need to make sure that you are looking at the total elapsed time to complete the activity. If you have a task that is estimated to take 5 days, based on an 8-hour day fully dedicated to that task, the actual duration estimate would be 10 days if the resource assigned to the task is only spending 4 hours a day on the task.

You also need to be aware of the difference between work days and calendar days. If your work week is Monday through Friday, and you have a 4-day task starting on Thursday, the duration for that task will be 6 calendar days, because no work will be done on Saturday and Sunday.

Figure 4.3 illustrates this situation. The same concept applies to holidays or vacation time.

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Figure 4.3: A 4-Day Task Separated by the Weekend

If you have estimated project activity durations in the past, you may not even think about talking about duration with your project team. This could lead to big problems down the road, so make sure that everyone doing estimating is in agreement up front as to whether the estimates will be provided in work days or calendar days. I recommend using work day duration estimates, as the project management software packages allow you to establish a calendar that accounts for non-work days and does not include these days when computing duration.

Now that we have a common understanding of duration, we are ready to discuss the different techniques used to create activity duration estimates.

Estimating Techniques

Where do those task duration estimates come from anyway? Although some cynics may tell you to use a dartboard to estimate duration, there are better ways. Some techniques are designed to provide a ballpark estimate with a wide margin of error when there is not a lot of hard data on the project available. The use of some estimating techniques is driven by the nature of the work involved in completing the task. There is no one right way to do task duration estimates. Just keep in mind that what is being done here is an estimate, and it is not a 100 percent guarantee of the length of time each task will actually take to complete.

Several techniques are used for activity duration estimates. We will take a look at three of the most commonly used and talk about some of the variables that can impact the accuracy of estimates made using each of these techniques.

Analogous estimating, or top down estimating.  Analogous estimating (top down estimating) is the use of actual durations from similar activities on a previous project. This is most frequently used at the early stages of project planning when you have limited information regarding the project. Although analogous estimating can provide an approximation of a task duration based on the length of similar activities, it is typically the least accurate means of obtaining an estimate. No two projects are exactly the same, and there is the risk that the project used to obtain the analogous estimates is not as similar as it appears.

Results from analogous estimating will be most accurate if the person doing the estimating is familiar with both projects and may be able to better understand differences that could impact the activity durations.

Expert judgment.  Expert judgment uses the people most familiar with the work to create the estimate. Ideally, the project team member who will be doing the task should complete the estimate. If all team members have not yet been identified, recruit people with expertise for the tasks you need estimated. How do you find people with the required expertise? If you do not have immediate knowledge of who the internal experts are, research the documentation on team members from similar projects or solicit input from your stakeholders. Ask for people who have completed a similar task on a previous project to assist with the estimates for your project.

The most accurate estimates based on expert judgment are those made by the person who will be completing the task, assuming that person can draw on past experience. One of the variables with project duration is the skill set and experience level of the team member performing the work. A duration estimate made by a senior tester will likely be shorter than that of a junior tester. If the person who is responsible for completing the task does not make the estimate, the project manager needs to make sure he or she validates the estimate with the person assigned to the task.

Quantitatively based durations.  Quantitatively based durations are used when a certain quantity of work is produced, and there is a formula to gauge duration. To apply quantitatively based durations, you must know the productivity rate of the resource performing the task or have a company or industry standard that can be applied to the task in question. The duration is obtained by multiplying the unit of work produced by the productivity rate. If a typical cable crew can bury 5 miles of cable in a day, it should take 10 days to bury 50 miles of cable.

This type of estimate can be very accurate for tasks that are repetitive and have a lot of productivity data to assure that the standard productivity rate accounts for variations in skill sets and conditions under which the work is performed. To determine if using quantitatively based durations is applicable to a given project, the project manager needs to understand the criteria for the company or industry standard.

Most projects use some combination of the estimating techniques. If some of the project tasks fit into an established productivity rate formula, they are ideal candidates for quantitatively based estimates; other tasks require the input of an expert familiar with the work.

Once you have determined which estimating technique(s) works best for your project, guide the team members as they work through all of the tasks on the network diagram and assign a duration estimate to each task as illustrated in Figure 4.4.

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Figure 4.4: Network Diagram with Task Duration

This will prepare you for the next process, schedule development.

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