During our advanced Primavera P6 classes I like to tease my students with the “weirdo” activity relationship and challenge them to find a practical situation for applying it. This relationship type is not available in scheduling programs like Microsoft Project so most of my students have never seen it before.
I am talking about the Start to Finish relationship, whereby the predecessor must start before the successor can finish. Unless a very large lag is used, the successor will start before the predecessors starts. Think about that for a moment. The successor starts first! The following illustration will make that statement rather obvious:
The predecessor (yellow bar) starts on the same day that its successor (green bar) finishes. If we add a lag duration the yellow bar will start before its successor finishes, so they will overlap. For example, a one-day lag would cause the predecessor to start the day before the successor finishes.
Well, you can probably see why we do not unleash this relationship type on beginners!
Other than showing off, one might wonder why the Start to Finish relationship is used at all. My favorite example is the pit crew that services a race car. The pit crew is the predecessor to the car arriving in the pits (the successor).
No, that is not a typo. I am saying the pit crew is the predecessor. Put another way, the race car is not supposed to pull into the pit lane until the pit crew is ready. The pit crew is therefore logically the predecessor because they, not the car, must be ready first.
At this point I am sure you are thinking, “why not make the race car the predecessor to the pit crew and use a Finish to Start relationship?” The issue is that we do not want the car to arrive early. So it is logical to say that the successor’s start date should determine the predecessor’s finish date. Hence, Start to Finish.
While I wish that race teams used Primavera software (official scheduler for Ferrari would be a very cool job) the reality is that one of my clients was already using this relationship type before taking my class. They work at a nuclear submarine facility on the East Coast. Similar to my example, they do not want the sub to show up until they are ready to perform maintenance. So maintenance is the predecessor.
The downside to this relationship type is that you can end up with some strange looking float values, as the backwards pass algorithm seems to get confused by what almost seems like reverse logic. We expect relationship lines to always be pointing to the right, after all. Other activities tied to the ones with the Start to Finish relationship can likewise behave rather strangely.
An alternative would be to use a Finish to Start relationship and put an “As Late As Possible” (ALAP) constraint on the predecessor. This way, the race car can be the predecessor but it will not finish until the pit crew is ready. Schedulers often use this constraint to avoid having materials or equipment delivered to the jobsite too early. Sort of like Just In Time manufacturing.
Perhaps it comes down to personal preferences. Some people dislike using Start to Finish relationships and others dislike using constraints. Owners in particular view constraints as an artificial device to sequester float. The contractor is in theory suppressing float on certain activities to keep the owner from using it. Fine, but when the door hardware shows up a year early perhaps they will reconsider!
Personally, I prefer to use the ALAP constraint, albeit with discretion. A side-effect of this particular constraint is that the predecessor(s) will not be driving. This can confuse some people. While other constraints sometimes have the same effect, this always happens with the ALAP constraint.
Try the Start to Finish relationship for yourself and see what happens!