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PM Essence

Dependency Relationships in Project Time Management


                                                                 -SSV Raghavan, PMP

Some time ago, I had an occasion to raise my dissenting voice in these columns, to argue that the instance of PTA in Project Procurement Management should be regarded as a variant of CPIF (Cost-Plus-incentive Fee) type of contract, and not one of FPIF (Fixed Price Incentive Fee) as mentioned in textbooks on the subject. Though there was no feedback in the columns of the PM Essence, the few phone calls I received in support, and the absence of any written rejoinders to my article, make me conclude that my viewpoint is accepted by the readers. This time I feel the urge to discuss another case, that of the dependency relationships in a Network Diagram – a subject belonging to the Knowledge Area of Project Time Management.

relationships in time management
We all know that in the notations F-S, S-S, F-F and S-F depicting the predecessor-successor relationships, the first alphabet stands for the Start / Finish of the predecessor activity and the second one that of the successor. Thus, the second phase of concreting waiting to begin till the first phase gets cured indicates a F-S Lag, while a relay runner taking over the baton from the previous lap runner is a case of a F-S Lead. In both these cases, the dependency is dictated by the predecessor, i.e., the start of the successor is dependent on the finish of the predecessor. Similarly for S-S relationship the example of preparation of a draft and its review, and for F-F, theinstance of preparation of various courses for a meal can be cited.
Difficulty however arises in getting a suitable example for visualizing a case of an S-F dependency. I have made an extensive Google search for this. While the relationship is correctly depicted diagrammatically with an arrow leading from the finish of a successor activity to the start of its predecessor, I was convinced by none of the examples cited ( as for instance the night watchman having to continue doing his duty till his reliever arrives to take over, or the pupils in a classroom - afflicted as they are with Students' Syndrome - continuing with their last minute preparations for a Class Test, and closing their books only after the teacher arrives, even if she happens to be late!). THESE EXAMPLES INVARIABLY INVOLVE THE FINISH, AND NOT THE START, OF THE PREDECESSOR ACTIVITY WHICH IS REQUIRED OF A S-F RELATIONSHIP, AND ARE THEREFORE ERRONEOUS. Consequently, I would regard them only as a VARIANT of the F-S relationship, but with a REVERSE DEPENDENCY, i.e., the Start of the successor driving and determining the Finish of the predecessor, unlike the cases cited above (concrete curing, relay runner).Moreover, the dependency in these cases cannot be defined upfront in a Network Diagram. One instance that was mentioned to me is that of payment being made in advance of the vendor making his supplies, but strictly speaking, will this not be case of a ROLE REVERSAL of the predecessor and successor, and therefore F-S again?


The only way I can conceptualize a S-F relationship is by having a CONSTRAINT (“Not Earlier Than” or “Not Later Than”) attached to the Finish of a Successor Activity which in turn would determine the Start of the Predecessor, having due regard to the respective durations of the two activities.


I shall be glad to have the views of the readers.