It may appear when scheduling tasks that a Finish-to-Start (FS) and negative lag is functionally the same as a Start-to-Start (SS) and positive lag, but they are not. Let’s take a closer look at these two approaches.
Many scheduling guidelines prohibit the use of negative lag. And we have already written about the drawbacks of negative lag in the our blog The Negatives of Negative Lag.
It is not the intent here to reiterate these arguments, but to warn that what appears the same in practice is not. Many schedulers think their schedule including a FS relationship and negative lag (negative FS) is the same as a SS and positive lag (positive SS). Well, the schedules look the same.
In fact both these schedules can have the same successor start dates, which makes them seem functionally the same. But they are not. A simple demonstration progressing the schedules reveals the dissimilar outcomes of these two activity relationships. Take caution the negative FS schedule is appreciably different from the positive SS schedule.
This article provides a simple demonstration showing the differing outcome when progressing a schedule including a negative FS instead of a positive SS relationship.
We have in Figure 1 our demonstration project created in Microsoft Project.
Figure 1
We have in this schedule three activities. Tasks A and B are connected by a Finish to Finish (FF) relationship and 2 day lag. Activities B and C are joined by a SS and 5 day lag. This schedule looks very similar to the schedule presented in Figure 2.
Figure 2
Again, tasks A and B are connected by a FF and 2 day lag. The only difference is that tasks B and C are linked by a FS and negative 5 day lag. If one inspects these two schedules they will find that activity C in both schedules has a start date 1/14/2019 and finish date 1/18/2019.
This leads schedulers to assume that both the negative FS and positive SS schedules work the same. And the negative FS may appear to provide a nicer fit to the schedule situation than one having a positive SS. So the allure of the negative FS is noted, but, caution, these two schedules have differing outcomes for the same progress.
In Figure 2 we enter status for the positive SS schedule.
Figure 3
Activity A has no progress. Activity B progresses one week, so the SS relationship between activity B and C is satisfied. B has begun so C can proceed on Monday as planned. In Figure 4 we move the status date forward one week and reschedule incomplete tasks to resume after the status date.
Figure 4
In this positive SS schedule activity B delays by a whole week due to its FF relationship with activity A. Task C however, is still scheduled to commence on Monday, as originally planned, which is our intention.
We progress the negative FS schedule in Figure 5.
Figure 5
Again, task A has no status and task B progresses one week. And it would seem that task C can, again, proceed on Monday. But that is not the case. Watch what happens when we schedule tasks to continue after the status date, Figure 6.
Figure 6
As in the previous example, task B delays an entire week due to its FF connection to task A. The difference, and problem, is that activity C also delays a whole week due to its negative FS relationship with activity B. This may not be our plan.
So seemingly similar schedules provide different outcomes in execution. In the first scenario task C is on target to begin on Monday as planned. In the second schedule task C is delayed one week, which may be an issue.
Although both outcomes may be agreeable to you, it’s important to understand the differing behavior. When task B is joined to task C with a positive SS the delay to the end of task B by task A does not affect the start of task C. However, when task B is connected to task C with a negative FS the delay to activity B by task A directly impacts the planned start of activity C.
Summary
Schedules that have the same start and finish dates for tasks, and only a minor relationship difference, may have dramatic outcomes in practice. The negative FS does not function the same as the positive SS, as demonstrated.
This may not stop your inclination to insert negative FS relationships, but you are now aware that this seemingly minor relationship change may have a significant impact on progress. And you may have to explain this delay, despite predecessor progress, to stakeholders.