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.
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