The PDM (Precedence Diagram Method) method of defining relationships between activities is a powerful tool. Often, however, Schedulers, sometimes unknowingly, misuse the options available when defining relationships between activities. In the illustration below, under Method 1, Activity “A” is related to “B” using Finish-To-Start (FS) and “B” is related to “C” using Start-To-Start (SS) with Lag = 0.
This may appear as a reasonable approach as often, the Activity Name description(s) may provide a false justification for this relationship. Unfortunately, the Forward and Backward pass doesn’t care about your Activity Name description. Therefore, trying to justify the use of Method 1 by using a “real world” situation fails as an argument to promote its use.
By observation, since “B” has a shorter duration than “C”, “B” should have a higher Total Float (TF) value than “C”. However, when calculated, the TF for “B” = 0. Under Method 1, the SS/Lag = 0 relationship between “B” and “C” is causing the critical path to flow through the only the Start of “B” resulting in an incorrect TF value for “B”.
By representing “B” as a critical activity, the result could lead to a possible misallocation of resources. Furthermore, if the correct TF value “B” is later discovered, then this could cause a loss in trust in the schedule.
The solution is to follow Method 2, i.e. “A” is related to “B” using FS and “A” is related to “C” using FS. Not only is this trap avoided but relationships between activities are more clearly defined.
Zümmer checks this anomaly with Report #37 – “Tasks With Start-To-Start Relationships And No Lag” reports all instances. In the illustration below, each Start-To-Start relationship with no lag is clearly and individually paired. In addition, the Activity Status of the predecessor and successor is listed along with each Original Duration, Total Float, type of Relationship and lag value between the two activities.
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