Short answer: Never.
Long answer: Summary tasks are a collection of one or more child tasks. By definition, a summary task doesn’t represent any real work, resources, or deliverables, so summary tasks are neither on nor off the critical path of a project. Children of summary tasks can be on the critical path, assuming those children are not summary tasks themselves.
So, when you set up a Microsoft Project plan and show the “Critical” field, you expect Project to tell you which tasks are critical and which aren’t, right? Well, not exactly. Take a look at the sample project below:
Microsoft Project is telling us that the first summary task, called “Summary A,” is on the critical path. This is partially accurate, since three of the four child tasks are actually critical. However, “Task 3” is not on the critical path, which means that while “Summary A” is not entirely off the critical path, it’s not entirely on the critical path either. When critical is a choice between Yes and No, summary tasks are somewhere in the middle.
Now, take a look at at the second summary task, called “Summary B”. Microsoft Project also thinks that it is on the critical path. This is completely inaccurate, because none of the child tasks underneath the summary are critical. If there are no critical children, then the summary task really shouldn’t be critical, but Microsoft Project tells us otherwise.
OnePager Pro accounts for this with Critical Path Segments, which are a more intelligent way to compute critical path for summary tasks. Instead of treating critical path as black and white, we consider the relationship between the child and summary tasks:
In the example above, “Summary A” shows a partial critical path indicator in red. The broken line maps to the children of “Summary A” that are on the critical path. When a child task is on the critical path, the dates of that child task float up to the summary task to show as critical. When a child task is not on the critical path, the corresponding portion of the summary task shows as non-critical.
In OnePager, critical path segments apply to all child tasks — not just the ones that you’ve specifically imported into your report. This means that you can create a Gantt chart consisting of only summary tasks, but still show the critical path based on the children downstream in the WBS, even if those child tasks aren’t showing in your report.
Here is OnePager’s interpretation of critical path for the Microsoft Project plan at the beginning of this article:
For the tasks under “Summary A”, we see which child tasks are critical and which aren’t. For the summary task itself, the broken line gives us an indication of where the critical path lies farther down in the WBS.
For the tasks under “Summary B”, there is no mention of critical path. Even though Microsoft Project thinks that “Summary B” is critical, OnePager knows better and omits the critical path indicators altogether.