Generally, in a CPM Network, lag values should be smaller than the duration values of either its predecessor or successor activity. In other words, lags should not cause gaps in the flow of activities through the CPM network. Unfortunately, this is a very easy mistake to make and worse, far more difficult to detect.
There are 4 possible cases where a non-overlapping lag may occur:
Case 1:
The Negative Lag is greater than the sum of the Predecessor’s Original Duration and the Successor’s Original Duration (Figure 1).
Suppose the Original Duration (OD) for Activities “A” and “B” are set to 5 Days each and the relationship is set to Finish-To-Start (FS) with a lag of -9 Days. This is overlapping since the sum of the durations of “A” and “B” is 10 Days.
Now Suppose, the OD for activities “A” and “B” are each reduced to 2 Days. Unless the relationship between “A” and “B” is changed, a non-overlapping condition will occur (Figure 1) since the absolute value FS lag of -9 Days is greater than the OD of “A” plus the OD of “B” (now at 4 Days). The gap between activity “A” and “B” is now 5 Days caused by the non-overlapping lag.
In P6, no Filter that can be constructed to check for this type of Non-Overlapping Lag.
Zümmer Analysis Report #31 – “Negative Lag Greater Than Sum of Both Original Durations” identifies the Non Overlapping lag condition “Neg Lag > Pred OD + Succ OD”. In the Figure 2 below, each non-overlapping relationship is clearly and individually paired. In addition, the Activity Status of the predecessor and successor is listed along with each Original Duration, the type of Relationship and lag value between the two activities.
Case 2:
A Positive Finish-To-Start lag is used between 2 Activities (Figure 3)
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 suppose the lag value is then changed to +7 Days. The gap between activity “A” and “B” is now 7 Days caused by the non-overlapping lag.
In P6, no Filter that can be constructed to check for this type of Non-Overlapping Lag.
Zümmer Analysis Report #32 – “Activities with Finish-To-Start Relationship & Positive Lag” identifies the Non Overlapping lag condition “FS with Positive Lag”. In Figure 4 below, each non-overlapping relationship is clearly and individually paired. In addition, the Activity Status of the predecessor and successor is listed along with each Original Duration, the type of Relationship and lag value between the two activities.
Case 3:
A Start-To-Start Lag is greater than the Predecessor’s Original Duration (Figure 5).
Suppose the Original Duration (OD) for Activities “A” and “B” are set to 15 Days and the relationship is set to Start-To-Start (SS) with a lag of 8 Days. This is acceptable since “A” and “B” overlap.
Suppose now, the OD for “A” is reduced to 5 Days. Unless the relationship between “A” and “B” is changed, a non-overlapping condition will occur since the SS lag of 8 Days is greater than the OD of “A” (now at 5 Days). The gap between activity “A” and “B” is now 3 Days caused by the non-overlapping lag.
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” identifies the Non Overlapping lag condition “SS Lag > Predecessor OD”. In Figure 6 below, each non-overlapping relationship is clearly and individually paired. In addition, the Activity Status of the predecessor and successor is listed along with each Original Duration, the type of Relationship and lag value between the two activities.
Case 4:
A Finish-To-Finish Lag is greater than the Successor’s Original Duration (Figure 7)
Suppose the Original Duration (OD) for Activities “A” and “B” are set to 15 Days and the relationship is set to Finish-To-Finish (FF) with a lag of 8 Days. This is acceptable since “A” and “B” overlap.
Suppose now, the OD for “B” is reduced to 5 Days. Unless the relationship between “A” and “B” is changed, a non-overlapping condition will occur since the FF lag of 8 Days is greater than the OD of “B” (now at 5 Days). The gap between activity “A” and “B” is now 3 Days caused by the non-overlapping lag.
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” identifies the Non Overlapping lag condition “FF Lag > Successor OD”. In Figure 8 below, each non-overlapping relationship is clearly and individually paired. In addition, the Activity Status of the predecessor and successor is listed along with each Original Duration, the type of Relationship and lag value between the two activities.
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