Effort, Duration, and Elapsed Time
In project management, but also whenever we want to estimate the work required to get a task done, there’s a time concept that is very important: the difference between Effort, Duration, and Elapsed Time.
Effort is the time it takes to actually get the work done.
Duration is the time from start to finish. A task may be started, but for whatever reason it gets interrupted, then the duration includes the working days in which work was not done on the task.
Elapsed Time is the same as Duration but also includes weekends and holidays (non-working days).
So, let’s take an example. You have two workers, A and B, and two tasks, 1 and 2. Task 1 only needs one person working two full days (for simplicity purposes, I’m using whole days and tasks. The same analysis works for hours, and even minutes! and projects).
So back to task 1. The Effort then is 2 days. The work starts on Monday and there are no interruptions, so the duration is also two days. As there are no weekends or holidays, then the Elapsed Time is also 2 days.
Task 2, requires two people, each working five days. So, the Effort is 2 people times 5 days or 10 days. The Duration, though, is five days, as the task started on Wednesday and finished the following Tuesday. However, the Elapsed Time is 7 days as it includes Saturday and Sunday. This example showcases the differences between the three concepts.
The graphic below shows these concepts.
So, when your manager asks “how long is it going to take to do a task?” determine what they are expecting. In most likelihood they are expecting either Duration or Elapsed Time. But if they need to know how much work is going into the task, it is key to know how many people are working on the task and for how long each.
These concepts apply also when you ask your team for estimates. Are they in sync with what you are thinking? Effort? Duration? Elapsed time? In many cases people think of duration when asked to estimate a task. Useful but not complete. Get clarity!
The examples here are extremely simplified. It does not include interruptions or conflicts with other higher priority items. It also assumes individuals work for a full day on a task whenever they work on the task. There’s no discussion on how to shrink the duration by adding workers. For example, if we add a third worker to Task 2, can we make the duration and elapsed times shorter?
Tools such as Microsoft Project can calculate this information for you but it is very important you understand the differences.
If you are leading a complex project or program, and using Map Days/CBPM, these times are not as important as they are details to be managed by the individuals/teams doing the work. Still, it is good to know the concepts as a senior PM. You may need to dig into the details of the tasks.
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