A key part of robust schedules in Primavera P6 are built by using logical relationships across the network of activities. In this article, we will discuss the different relationship types in Primavera P6 available to schedulers, offering examples of their use. Better still, we have also produced a short video where users can visually see the varying differences between the relationship types used in Primavera P6.
In Primavera P6, a relationship refers to a link between two activities. This link describes the flow of work between tasks, as shown in Figure 1.
Figure 1
For Example:
When a house is being built, work cannot start on the roof until the walls are complete.
Primavera P6 uses the terms predecessor and successor to describe the relationship between two activities, as shown in Figure 2.
Figure 2
There are four relationship types to choose from in Primavera P6:
Finish to Start (FS)
The most commonly used relationship is (FS).
For Example:
When build walls have Finished the installation of the roof can Start, as shown in Figure 3.
Figure 3
Start to Start (SS)
SS is used to describe a starting dependency between tasks.
For example:
The Site work activity cannot Start Until Safety Management Starts, as shown in Figure 4.
Figure 4
This type of activity is commonly used with Start milestones, as shown in Figure 5.
Figure 5
Finish to Finish (FF)
FF is used to describe a Finishing dependency between tasks. One activity cannot Finish until another activity Finishes.
For example:
Project Management cannot Finish until the site work is Finished, as shown in Figure 6.
Figure 6
This relationship is commonly used with Finish Milestones, as shown in Figure 7.
Figure 7
Start to Finish (SF)
This is a somewhat confusing relationship type and is the least used of all the activity relationships. It states that an activity cannot Finish until its’ successor Starts, as shown in Figure 8
Figure 8
Most Industry best practice guidelines actively discourage its’ use. There are very few examples of a truly, practical purpose for this relationship therefore we recommend that you avoid using it, as shown in Figure 9.
Figure 9
Summary
A large part of the success of any project is predominantly achieved by adhering to a realistic and accurate schedule that makes logical steps through the network. Building a schedule that is transparent by using the FS relationship will represent the sequence of activities realistically.
SS and FF relationships are used, but they are usually tied to Start and Finish Milestones.
The SF relationship does exist in Primavera P6, but owing to its confusing nature it is very rarely used. We wholly recommend, along with most Project Management guidelines, that this relationship is not used.