Variation in Activity Duration

Activity duration is a random variable. Because we cannot know what factors will be operative when work is underway on an activity, we cannot know exactly how long it will take. There will, of course, be varying estimates with varying precision for each activity. One of your goals in estimating activity duration is to define the activity to a level of granularity so that your estimates have a narrow variance—that is, the estimate is as good as you can get it at the planning stages of the project. As project work is completed, you will be able to improve the earlier estimates of activities scheduled later in the project.

There are several causes of variation in the actual activity duration:

Varying skill levels. Our strategy is to estimate activity duration based on using people of average skills assigned to work on the activity. In actuality, this may not happen. You may get a higher- or lower-skilled person assigned to the activity, causing the actual duration to vary from planned duration. These varying skill levels will be both a help and a hindrance to us.

Unexpected events. Murphy lives in the next cubicle and will surely make his presence known, but in what way and at what time we do not know. Random acts of nature, vendor delays, incorrect shipments of materials, traffic jams, power failures, and sabotage are but a few of the possibilities.

Efficiency of work time. Every time a worker is interrupted, it takes more time to get up to the level of productivity prior to the time of the interruption. You cannot control the frequency or time of interruptions, but you do know that they will happen. As to their effect on staff productivity, you can only guess. Some will be more affected than others.

Mistakes and misunderstandings. Despite all of your efforts to be complete and clear in describing the work to be performed, you simply will miss a few times. This will take its toll in rework or scrapping semicom-pleted work.

Common cause variation. Apart from all of these factors that can influence activity duration, the reality is that durations will vary for no reason other than the statistical variation that arises because the duration is in fact a random variable. It has a natural variation, and nothing you do can really decrease that variation. It is there, and it must be accepted.

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