Predecessor and Successor Logic
Activities that are logically related within a schedule network are referred to as predecessors and successors. A predecessor activity must start or finish before its successor. The purpose of a logical relationship, or dependency, is to depict the sequence in which activities occur. Such relationships state when activities are planned to start and finish in relation to the start and finish of other activities. A logic relationship therefore models the effect of an on-time, delayed, or accelerated activity on subsequent activities. Relationships between activities can be internal—that is, within a particular schedule—or external—that is, between schedules.
Logical relationships between activities identify whether they are to be accomplished in sequence or in parallel. A sequence of activities is a serial path along which one activity is completed after another. Activities can also be accomplished in parallel or concurrently. A logic relationship linking a predecessor and successor activity can take one of three forms: finish-to-start, start-to-start, and finish-to-finish.
A finish-to-start (F-S) relationship is the most straightforward logical link between a predecessor and successor. In it, a successor activity cannot start until the predecessor activity finishes, creating a simple sequence of planned effort. This logical relationship is the default in most scheduling programs. In figure 8, the installation of roof decking cannot begin until the installation of roof trusses finishes. Note that the installation of the roof decking does not necessarily need to start once the installation of the roof trusses finishes, but it cannot start until the trusses are installed. The “install roof decking” activity may have other predecessors that push its start date further into the future.
Figure 8: A Finish-to-Start Relationship
A start-to-start (S-S) relationship dictates that a successor activity cannot start until the predecessor activity starts. In the example in figure 9, the application of wall finishes cannot start until the application of drywall texture starts. The S-S relationship does not dictate that wall finishing must start at the same time that drywall texturing starts, but it does indicate that it cannot start until drywall texturing starts. The “apply wall finishes” activity may have other predecessors that push its start date further into the future.
Figure 9: A Start-to-Start Relationship
A finish-to-finish (F-F) relationship dictates that a successor activity cannot finish until the predecessor activity finishes. In figure 10, final grading cannot finish until forming and pouring the driveway has finished. Again, the F-F relationship does not dictate that landscaping must finish at the same time as pouring the driveway finishes, but it cannot finish until the pouring of the driveway finishes. The final landscaping activity may have other predecessors that determine its finish date, or it may finish later than the driveway pouring activity.
Figure 10: A Finish-to-Finish Relationship
The start-to-finish (S-F) link is a theoretical, fourth combination of logical links between predecessor and successor. It has the bizarre effect of directing a successor activity not to finish until its predecessor activity starts, in effect reversing the expected flow of sequence logic. Its use is widely discouraged because it is counterintuitive and it overcomplicates schedule network logic. Examples of activity sequences used to justify the existence of an S-F relationship can usually be rewritten in simple F-S logic by either subdividing activities or finding more appropriate F-S predecessors within the network.
The majority of relationships within a detailed schedule should be finish-to-start. Finish-to-start relationships are intuitive because most work is accomplished serially in that order. Moreover, F-S relationships are easy to trace within a schedule network and clearly indicate to management which activities must finish before others begin and which activities may not begin until others have been completed. F-S relationships are implemented most easily where work is broken down to small elements.
Start-to-start and finish-to-finish relationships, in contrast, imply parallel or concurrent work. S-S and F-F relationships represent a valid technique for modeling the overlapping of activities. As such, they may be more predominant in schedules whose detail has not yet evolved. However, an overabundance of these relationships in detail schedules may suggest an overly optimistic or unrealistic schedule or shortcuts that have been taken in modeling activities and logic. Particularly in a detailed schedule, their overuse may impair the usefulness of the schedule by, for example, complicating the identification of the critical path. S-S and F-F relationships are also prone to producing unintentional “dangling” relationship logic, an error that we describe later in this best practice.