Inputting the tasks and the relationships among tasks in the scheduling software program can be a meticulous process. Primavera P6 has a helpful way of viewing these logical relationships to aid the insertion and verification of activity relationships. It is also important for the scheduler to understand the four logical relationships between activities. This article describes these logical relationships with examples to help better understand the implementation of each logical relationship. Also discussed are lead and lag activity relationships.
Primavera P6 uses what may be considered triangulation to assist with the input of activity relationships among one another. Most project scheduling programs consider the predecessor successor relationships between activities. Primavera P6 takes this a step further in the activity relationship view by displaying the current activity, the predecessor to that activity, and then the successor to that same activity. Therefore, you have displayed before you the predecessor, current, and successor activities. This provides a “birds eye” view of the current activity’s place in the Gantt chart.
The figure below shows the relationship view in Primavera P6 that describes the current activity: (A1060) Install Slab Plumbing. Its predecessor (A1050), which is also displayed, is Install Foundation Forms. Finally, the successor (A1070), Pour and Float Slab Concrete, is also shown.
Each activity can have a finish-to-start, start-to-start, finish-to-finish, or start-to-finish relationship to the following accompanying activity.
The finish-to-start relationship is by far the most commonly used relationship between activities, and also the easiest to understand logically.
In the finish-to-start relationship the successor activity cannot start until the predecessor completes. An example would be that the flooring must be completed before the frame of the building can go up.
The start-to-start relationship is when two tasks must begin at the same time. For example the safety plan must be implemented at the start of the construction work. Another way of describing this is that the start of task A controls the start of task B. In other words, for our example, the implementation of the safety plan controls the start of task B.
A finish-to-finish relationship is when two tasks must finish at the same time. An example of a finish-to-finish relationship is that testing must finish before the completion of the accompanying documentation. Another way of expressing this is: the finish of A controls the finish of B.
A start-to-finish relationship is the least common and, probably, the most confusing. It means that B may not finish until A starts. A possible example is Task B, running a generator to power a construction site. Task A may be turning on a power line to the site, while task B again, is running the generator. Once A, the power lines are started, then B, running the generator, may finish.
Leads and Lags
A lead indicates that an activity can start before its predecessor activity is completed. A lag is a period of time inserted between activities, such as, waiting for concrete to cure before building the frame of a house on the concrete flooring. This lag describes the time between these two activities when no project work is done.
Using the correct relationships among activities is an important step to describing a project. Primavera P6 aids the user by displaying the current activity along with both the predecessor and successor activities. This is particularly helpful when trying to input the logical relationship (finish-to-start, start-to-start, finish-to-finish, and start-to-finish) between activities, as well as, the respective lead or lag start times between activities.