Tasks With Finish-To-Finish Relationships And No Lag – The Official Site for Zümmer

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 “B” is related to “C” using Finish-To-Finish (FF) 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.

*The Forward and Backward Pass doesn’t care about your Activity Name description.*

In Figure 1 above, by observation, since “C” has a shorter duration than “B”, the Early Start (ES) for “C” is misrepresented since “C” can start at the completion of “A”. Furthermore, the Total Float (TF) is equally misrepresented since the Early Finish (EF) is the same as “B”.

If there is no lag (say this to yourself a few times) between “B” and “C”, then the finish of both “B” and “C” are equally responsible for starting “D”. Neither “B” or “C” have priority over controlling the start date of activity “D” with respect to logic. With respect to Duration however, activity “B” alone controls the start date of activity “D”. Therefore, the correct approach is to relate “B” FS to “D”.

In Method 1 above, activity “C” is behaving as if it was constrained using the “As Late as Possible” option. By misrepresenting the ES of “C”, the result could lead to an inadvertent impact to the Project Completion date by starting “C” too late. This could cause a loss of trust in the schedule. The solution is to follow Method 2, i.e. “B” is related to “D” using FS. Not only is this trap avoided but relationships between activities are more clearly defined.

After all this, if you’re still not convinced and still feel the need to use the FF 0, then consider re-assigning the scope of work between Activities “B” and “C”. Consider separating the scope of the tasks so that the finish portion of Activity “C” is now included as part of Activity “B” and the start portion of Activity “C” starts at the completion of Activity “A”. Normalizing CPM Network logic to the basic Finish-to-Start relationships avoids confusion and possible erroneous Start/Finish dates and Total Float values.

Zümmer checks this anomaly with Report #35 – “Tasks With Finish-to-Finish Relationships And No Lag” reports all instances. In the illustration below, each Finish-To-Finish 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. Relationships with lower Total Float values have a higher resolution priority over relationships with a higher Total Float value.