A question that often results in much debate in online forums, is one about what constitutes – by way of an example – a Start to Finish precedence relationship in project management. I will clarify the differences in precedence relationships in this post.
Precedence relationships are the associations (links) formed between tasks on a project WBS (Work Breakdown Structure). There are four basic types of precedence relationships, these being:
- Finish to Start (FS) – by far the most common you will see, where the preceding activity must finish before the succeeding one can start;
- Finish to Finish (FF) – where the succeeding activity cannot finish until the preceding task finishes;
- Start to Start (SS) – the opposite of FF, where the succeeding activity cannot start until the preceding task starts;
- Start to Finish (SF) – you very rarely see this type; it is where the succeeding task cannot finish until the preceding task has started.
Let’s now look at some examples for each of these. If you are rewiring the 240v supply in a building. The preceding task could be to isolate the mains power feed. This task must finish before the succeeding task of removing the old wiring begins (unless you want to get electrocuted), so this is an FS relationship.
A project to install a large water tank might have a succeeding task to fill the tank with water, but this cannot be completed until the preceding task of fitting the overflow system to the top of the tank is completed, so this is an FF relationship. You could structure this as an FS relationship but the total time then goes from 4 days to 7 days to complete the work.
An example of an SS relationship is where there is a project to sculpt a set of hedgerows for a botanical exhibition. The clearing up of the clippings can be done in parallel to the trimming, but cannot it start before it; again you could structure this as an FS or FF.
Now comes the tricky one, the Start to Finish relationship. The conversations online offer lots of examples, not always correct as some are simply Finish to Start relationships in reverse order. E.g. ‘Remove wiring’ cannot finish until ‘Isolate mains supply’ has started. This suggests you can start to remove the wiring with the main supply still active, as that task is started but not finished – sounds like a recipe for fried chicken.
For SF relationships the preceding activity must have started in order for the succeeding activity to complete. I can recall an example from a project to replace a booking system. The new system came online alongside the old one. The preceding task was to ‘Capture forward bookings (new system)’, once this started, the succeeding task, to ‘Cease forward bookings (old system)’ could then be completed; this is an SF relationship. This could be treated as an FS relationship, however, there is a risk that service would be disrupted in the gap between ceasing and starting bookings in the systems – not fried chicken, but more likely frazzled customers.
Note that this is different from say a scenario where you go-live with new a new booking system (preceding) and then decommission the old booking system (succeeding); this would be an FS relationship, as you would be starting the processes of decommissioning, not finishing it; whereas ‘Cease forward booking (old system)’ is completion of an activity.
In summary SF relationships are so rare, that for the most part, we need not concern ourselves with them. An easy way to remember them is to think of a hostage situation on a bridge. The ransom payer (succeeding) is not going to complete handover of the money to the hostage taker (preceding) until they see the hostage start walking towards them.