Hidden Open Ends: No Finish Side Successor/No Start Side Predecessor

Introduction

The Precedence Diagram Method (PDM) provides several ways to define relationships between activities. However, schedulers may incorrectly use these options in ways that allow logic to bypass one end of an activity. Two common examples:

1. No Finish-Side Successor: no relationship carries logic out of the activity’s finish.
2. No Start-Side Predecessor: no relationship carries logic into the activity’s start.

These two patterns mirror each other, so this article covers both. It explains:

1. how each pattern creates an activity that is open-ended in practice,
2. why P6’s standard filters don’t catch them, and
3. how Ƶümmer detects, identifies and reports them.

Case 1: No Finish-Side Successor

In Figure 1, Task D is a successor to Task C using a Finish-to-Start (FS) relationship and is a predecessor to  Task F using a Start-to-Start (SS) relationship.

Task D has both a predecessor and a successor activity, so it does not violate the “No Open Ends” rule. Accordingly, the P6 Global filters “Activities without Predecessors” and “Activities without Successors” return no results.

However, Task D’s only successor is linked from its start. No logic leaves Task D’s finish, so the finish behaves as if it were open-ended.

Increasing Task D’s Original Duration (OD) and recalculating the schedule gives these results:

1. OD from 10 to 12 days: Task D’s Total Float (TF) does not change.
2. OD of 17 to 22 days: Task D’s TF value decreases, but Task F’s start and finish dates do not change.
3. OD of 23 days or more: Task D becomes critical and finishes after 29-Nov-26. Task F and Finish Milestone G still show a finish of 29-Nov-26.

You can’t build a P6 filter that finds this condition because there are no options available to construct the specific query. Ƶümmer identifies this anomaly in Analysis Report #14, “Activities with No Finish Side Successor,” (Figure 2).

Case 2: No Start-Side Predecessor

In Figure 3, Task D is a successor to Task C using a Finish-to-Finish (FF) relationship with a 5-day lag and is a predecessor to Task F using a FS relationship.

As in Case 1, Task D has both a predecessor and a successor, so P6’s open-ends filters returns no results.

However, Task D’s only predecessor is linked to its finish. No logic enters Task D’s start, so the start behaves as if it were open-ended.

Increasing Task D’s OD and recalculating the schedule gives these results:

1. OD from 10 to 31 days: No dates or TF values change for Task D or any other activity.
2. OD of 32 to 41 days: Task D’s TF value decreases, but no other activity’s dates change.
3. OD of more than 42 days: Task D starts on the Data Date and becomes critical. Task F is affected; however, Start Milestone A and Tasks B and C keep their dates, and their TF value increases accordingly.

You can’t build a P6 filter that finds this condition because there are no options available to construct the specific query. Ƶümmer identifies this anomaly in Analysis Report #15, “Activities with No Start Side Predecessor” (Figure 4).

Conclusion

In a CPM network, logic should flow through every task like water through a pipe. During the forward pass, at least one relationship must carry logic into the start and out of the finish of each task. During the backward pass, at least one relationship must carry logic into the finish and out of the start of each task. Checking both conditions helps confirm—although it does not guarantee—that the CPM network conforms to industry best practice rules and guidelines.

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