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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:

No Finish-Side Successor: no relationship carries logic out of the activity’s finish.
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:

how each pattern creates an activity that is open-ended in practice,
why P6’s standard filters don’t catch them, and
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:

OD from 10 to 12 days: Task D’s Total Float (TF) does not change.
OD of 17 to 22 days: Task D’s TF value decreases, but Task F’s start and finish dates do not change.
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:

OD from 10 to 31 days: No dates or TF values change for Task D or any other activity.
OD of 32 to 41 days: Task D’s TF value decreases, but no other activity’s dates change.
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.

Understanding how Negative Total Float values can occur without any constraints

Introduction

Negative Total Float values are commonly interpreted as an indication that a Project is not meeting its completion requirements. This is often due to the presence of an imposed constraint such as “Finish-On-or-Before” or “Must Finish By” on a task, milestone or the Project in the CPM network. When Negative Total Float values occur, it typically signals the need for corrective action(s) to recover from any resulting delays or impacts.

Negative Total Float Values Without Constraints

While constraints are the usual cause of negative Total Float values, it is possible for negative Total Float values to occur even in their absence. This scenario can arise from the way calendars are assigned to different tasks and milestones within a schedule.

Illustrative Example

Consider the Project’s “Zum_NTFL” CPM network shown in Figure 1 above. For clarity, the Early Start, Early Finish, Late Finish and Calendar assignment columns are displayed. In addition, the Date Format is expanded to show the Time with AM/PM. Finally, the Data Date is set to Monday, June 1 at 8:00 AM.

Activity A1000 – “Milestone A” is a Start Milestone and is assigned to the PC – Standard 5-Day Work Week Calendar.
Activity A1010 – “Task B” is assigned to the PC – Standard 5-Day Work Week calendar and has an Original Duration of 2 Days, resulting in an Early Finish of Tuesday, June 2 at 5:00PM.
Activity – A1020 – “Task C” is assigned to the PC – Standard 7-Day Work Week calendar and has an Original Duration of 4 Days, resulting in an Early Finish of Saturday, June 6 at 5:00PM.
Activity A1030 – “Milestone D” is a Finish Milestone and is assigned to the PC – Standard 5-Day Work Week calendar.
The 7-Day and 5-Day Work Week calendars are configured so that the Detailed work hours/day setting include 8 Work hours per day from 08:00 thru 12:00 and from 01:00 thru 05:00.

Since “Milestone D” is assigned to the PC – Standard 5-Day Work Week calendar, the calendar’s setting does not allow for a Saturday finish. As a result, the Early Finish for “Milestone D” is Monday, June 8 at 08:00 AM. Note that the time is 08:00 AM (instead of 5:00PM) because that is the earliest available date/time in the PC – Standard 5-Day Work Week calendar.

During the Backward Pass, the Late Finish for “Milestone D” begins at latest available Finish Date based on the PC – Standard 5-Day Work Week calendar which is Friday, June 5 at 5:00 PM. Keep in mind that with respect to the PC – Standard 5-Day Work Week calendar, 08-Jun-26 08:00 AM and 05-Jun-26 05:00 PM are effectively the same point in time.

Since “Task C” is a direct predecessor to “Milestone D”, “Task C”’s Late Finish is also calculated to Friday, June 5 at 5:00PM. However, that date is 1 day earlier than its Early Finish of Saturday, June 6 at 5:00PM. Therefore, the Total Float value for “Task C” is -1 Day and the Backward Pass propagates this negative value through the remaining CPM network.

Accumulation of “False Negative” Total Float values

In complex CPM networks with numerous activities and multiple calendars, this effect can compound, resulting in what can be classified as “False Negative” Total Float values. These values do not necessarily reflect actual schedule problems but are rather a result of calendar misalignment.

This anomaly is also noted in the “Warnings” section of the P6 Schedule log text output file when a Finish Milestone has a different calendar than its predecessor activity (See Figure 2 below).

Solution

To resolve the issue illustrated above, one simple solution is to reassign “Milestone D” to the same 7-Day Work Week calendar used for “Task C”. Once this adjustment is made, recalculating the schedule will eliminate the negative Total Float value, restoring accurate Total Float values throughout the CPM network.

Non-Overlapping Lags

In a CPM network, a lag’s value should be less than the Original Duration of its predecessor and successor activity pair so that it doesn’t open a gap in the flow of logic. When a lag exceeds the predecessor’s or successor’s assigned Duration, a non-overlapping condition can occur.

Many schedule specifications prohibit using lags in place of activities. At first glance, only a Finish-to-Start relationship with a positive lag appears to break that rule (Case 2 below). On closer inspection, Cases 1, 3, and 4 break it too: any lag that opens a gap between the predecessor and successor activity pair has the same effect as the more familiar Case 2.

These gaps are easy to create and hard to spot. A non-overlapping lag can occur in four ways and are addressed in the Zümmer Analysis Selected Report(s) box shown in Figure 1 below:

Case 1: The Negative Lag is greater than the sum of the Predecessor’s Original Duration and the Successor’s Original Duration (Figure 2).

Suppose the Original Duration (OD) for Activities “A” and “B” are set to 5 Days each, with a Finish-To-Start (FS) relationship and a lag of -9 Days. This is an acceptable overlap, because the absolute value of the lag (9 Days) is less than the combined OD of “A” and “B” (10 Days).

Now suppose the OD of both activities are reduced to 2 Days. Unless the relationship changes, a non-overlapping condition occurs (Figure 2): the absolute value of the FS lag (9 Days) exceeds the combined OD of “A” and “B” (4 Days), leaving a 5-Day gap between them.

In P6, no filter can be constructed to check for this type of non-overlapping lag. Case 1 is the least common of the four, because most schedule specifications prohibit negative lags outright.

Zümmer Analysis Report #31 – “Negative Lag Greater Than Sum of Both Original Durations” flags this condition as “Neg Lag > Pred OD + Succ OD.” Figure 3 below lists each non-overlapping relationship as an individual pairing, showing the Activity Status and Original Duration of both the predecessor and successor, plus the relationship type and lag.

Case 2: A Positive Finish-To-Start lag is used between 2 Activities (Figure 4).

Suppose the Original Duration (OD) for Activities “A” and “B” are set to 7 Days each and the relationship is set to Finish-To-Start (FS) with a lag of -2 Days. This is overlapping since the sum of the durations of “A” and “B” is 14 Days.

Now change the lag to +7 Days. A positive Finish-To-Start lag pushes “B” away from “A”, opening a 7-Day gap. This can be resolved by replacing the lag with an activity whose Original Duration equals the lag value.

In P6, no filter can be constructed to check for this type of non-overlapping lag. Case 2 is often used to force a successor’s start date, especially when the predecessor and successor are assigned to different calendars.

Zümmer Analysis Report #32 – “Activities Finish-To-Start Relationship with Positive Lag” flags this condition as “FS with Positive Lag.” Figure 5 below lists each non-overlapping relationship as an individual pairing, showing the Activity Status and Original Duration of both the predecessor and successor, plus the relationship type and lag.

Case 3: A Start-To-Start Lag is greater than the Predecessor’s Original Duration (Figure 6).

Suppose the Original Duration (OD) for Activities “A” and “B” are set to 15 Days each, with a Start-To-Start (SS) relationship and a lag of 8 Days. This is acceptable, because “A” and “B” overlap.

Now reduce the OD of “A” to 5 Days. Unless the relationship changes, a non-overlapping condition occurs: the SS lag (8 Days) exceeds the OD of “A” (5 Days), leaving a 3-Day gap between the two activities.

In P6, no Filter that can be constructed to check for this type of Non-Overlapping Lag.

Zümmer Analysis Report #33 – “Start-To-Start Relationships & Lag Greater Than Predecessor’s Original Duration” flags this condition as “SS Lag > Predecessor OD.” Figure 7 below lists each non-overlapping relationship as an individual pairing, showing the Activity Status and Original Duration of both the predecessor and successor, plus the relationship type and lag.

Case 4: A Finish-To-Finish Lag is greater than the Successor’s Original Duration (Figure 8).

Suppose the Original Duration (OD) for Activities “A” and “B” are set to 15 Days each, with a Finish-To-Finish (FF) relationship and a lag of 8 Days. This is acceptable, because “A” and “B” overlap.

Now reduce the OD of “B” to 5 Days. Unless the relationship changes, a non-overlapping condition occurs: the FF lag (8 Days) exceeds the OD of “B” (5 Days), leaving a 3-Day gap between the two activities.

In P6, no Filter that can be constructed to check for this type of Non-Overlapping Lag.

Zümmer Analysis Report #34 – “Finish-To-Finish Relationships & Lag Greater Than Successor’s Original Duration” flags this condition as “FF Lag > Successor OD.” Figure 9 below lists each non-overlapping relationship as an individual pairing, showing the Activity Status and Original Duration of both the predecessor and successor, plus the relationship type and lag.