Dangling Logic

“It is the loose ends with which men hang themselves.”

- Zelda Fitzgerald

In the DCMA 14 Point Assessment, there are checks for missing predecessors and missing successors. In the DECM, the test for dangling logic is an improvement to these checks. What is dangling logic, and why does DCMA care about it? More importantly, why should you care about it?

When you add a particular activity to a schedule network, there generally should be another activity or milestone that influences its start date. Typically, the other activity is linked via a finish-to-start (FS) relationship. The predecessor is said to drive the start date of the successor. Another relationship that drives the start date of an activity is a start-to-start (SS) link. When an activity has at least one FS or SS predecessor, its start date will be driven by the predecessor.

But when an activity has no predecessor, or when its predecessors are either finish-to-finish (FF) or start-to-finish (SF), nothing will be driving the start date. We call this an open start. In a network diagram, no arrows will be attached to the left side of the box. The left side of the box has no anchor, so we think of it as dangling.

The same is true of the right side of the box. When an activity has no finish-to-start successors and no finish-to-finish successors, no arrow leaves the right side of the box. The finish date of the activity isn’t driving any successors, so the activity is dangling because it has an open finish. If the activity has successors, but they are start-to-finish or start-to-start, the task is still dangling because the finish date isn’t driving anything else.

Why is dangling logic a problem?

Suppose an activity has an FS successor. When it is delayed, the successor’s date will move as well. There will be a visible consequence of the delay. But when it has an open finish, it’s a different story. If the task takes longer than expected, the effects of that delay will be much harder to spot. Budgets will (quietly) be exceeded. Similarly, a task with an open start might start two weeks early and finish two weeks later than expected with no visible impact on the schedule but an extra two weeks of cost.

As the quote at the start of the chapter alludes to, dangling logic is an easy way to hang yourself.

Next steps

For each task that is flagged as a dangling task:

  • If no predecessors are start-to-start or finish-to-start, find the real-world predecessor that affects the start date of the task and link it properly to the task. We recommend using a finish-to-start predecessor.

  • If no successors are finish-to-finish or finish-to-start, find the real-world successor that would be affected by this task and link it properly to this task. We recommend using a finish-to-start successor.

Does the 14 Point Assessment include this check?

14 Point

The 14 Point Assessment checks for missing predecessors and missing successors and will catch dangling activities in some cases but miss them in others. For example, an activity with one FF predecessor and one FS successor will pass the 14 Point Assessment because it has both a predecessor and a successor. It will fail the DECM check for dangling logic because it has an open start.

Does Analyzer check for dangling logic? 


Yes. We’ve supported a check for dangling tasks since April 2009.