Tuesday, November 29, 2016

Work on summary task different from the sum of sub-tasks work! What is happening??

Back to the basics, I'll talk today about a support case I got today in MS Project. Always good to go back to scheduling challenges.
The issue? When running odata reports in Excel, a customer tells me that the work on the summary task is greater than the sum of the sub-tasks work.

Something like this:

2 tasks with 4 and 2 days of work, the work on the summary task should be 6 days and not 12 right? Here were my first thoughts that I submitted to my customer :

  • a filter might have been applied so some of the sub-tasks might not be visible. NO!
  • the tasks might not be sorted along the ID, so some sub-tasks might be located at another place in the WBS. NO!
  • the summary task might be manually scheduled. NO! (anyway this wouldn't have helped since even if it was the case, the sum should have been correct).

Any idea? Guess what, I've been telling for 10 years to my customers NOT TO ASSIGN RESOURCES ON SUMMARY TASKS, but I just didn't think about it! What puts me on the wrong way was the screenshot I received from the customer, similar to the one above: no resource is displayed near the summary task bar. Unfortunately, I didn't remember that the summary task bar style does not display any information around the bar.

So here is what happened:

Without any resource on the summary task, you get 6 days of work. If you add a resource on the summary task with the default unit (100%), on a 6-days duration task, you'll get 6 days of additional work, which makes 12 days in total.

Here is how 2 + 4 = 12 !

So keep that in mind, this is really a worst practice! Not convinced? I could give you several examples. Quite often, the use case about assigning resources of summary task is when you don't want to assign the same resource on 10 tasks, you think this is simpler to directly assign it on the summary task. Here is a simple reason why you should not: it can create ghost over or under allocations

Here is a use case with a resource assigned on 2 task with a positive lag. You can see that I have a duration of 8 days, but 6 days only of work since I have a 2-days lag.

If I assign the resource on the summary task which has no lag, the work is calculated based on the summary task duration (8 days). I thus have 8 days of work, which is not what I want.

Similarly, if I put a negative lag, meaning that the task cannot start earlier than 2 days before the predecessor is finished, I get a 4-days duration but 6-days work since 2 days are in parallel. Moreover, I have an over allocation since resource A is assigned on both task the 2 first days.

If you assign resource A on the summary task which is 4-days duration, you get 4 days of work instead of 6 and no over allocation.

Hope you are now convinced that there takes a lot of verifications when assigning resources of summary tasks. You might anyway need or want to do it (even if I do recommend not to do it), but be aware of the traps and consequences.

Have a good scheduling!

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