How Uncertainty Worksby Kevin Thompson, Ph.D, PMP, CSP
How Uncertainty WorksAll estimates are subject to uncertainty, and project schedules are no exception. A schedule consists of a set of tasks, which are executed at times dictated by dependencies and resources. The simplest schedule, consists of a set of tasks that are executed one after the other, and we’ll look more at this case below. The schedule for a project contains uncertainty because the estimated effort or duration of each task has some uncertainty associated with it. We would really like to know exactly how long a task will take. If we can’t know that, we would at least like to know how much uncertainty is associated with the task. Unfortunately, we will never know the first, and usually will never know the second, either. (The exception to the latter rule is for tasks that are repeated, identically, enough times for us to collect meaningful statistics about them.) In this article, we will look at uncertainty: Why it exists, how it behaves, how it accumulates, how to reduce it, and how to cope with it.
- Remodel Kitchen
- Remodel Bedroom
- Remodel Living Room
- The most likely case is 3 days.
- The best case is 1.5 days, which is 1.5 days under the estimate.
- The worst case is 6 days, which is 3 days over the estimate.
- Remove old carpet
- Paint room
- Cut new carpet to fit
- Install new carpet
- For fixed-schedule and fixed-scope projects, add buffer time in the schedule. This works for low-uncertainty projects, especially those that repeat the same type of work many times.
- For fixed-schedule projects, use an agile process such as Scrum, and adjust scope in a planned way to meet the schedule. This is an effective way to conduct a project when estimates are poor, and scope is poorly-defined and changes frequently, while still allowing for planning and a useful degree of predictability.
- For unscheduled projects with unknown scope, uncertainty is very high, and planning is not possible. In this case, a strategy such as Kanban, which focuses on constraining work-in-progress, is effective.