We’ve produced a short video, ‘Using Relationships in Primavera P6’ where you can get a feel for the different relationship types in P6 and tips for assigning them to a schedule. We get a number of questions regarding the relationship types in Primavera P6 and how to use them and when to use which type. While there’s no hard and fast rules in how they’re used, there are some generally accepted rules, especially for those new to P6.
A summary of the video is below:
One important item when creating a schedule in Primavera P6, is to build logical relationships between activities, as shown below in Figure 1.
All activities in a schedule should have an allocation of their dependencies to another activity in the schedule. In this article, we discuss the types of relationships, logic, and using relationships in Primavera P6.
What is a Relationship/Dependency?
A relationship (dependencies are also known as relationships) is the universal term for a connection that shows how one activity is dependent on another in a schedule. For example, the Start of activity B is dependent upon the completion Finish of activity A, as shown in Figure 2.
Logic is the logical sequencing of relationships. For example, if a house was being constructed, the roof is dependent on the completion of the walls and the walls are dependent on the completion of the foundations as shown below in Figure 3.
It would be physically impossible to build the house in any other logical sequence hence the term ‘Logic.’
In our example, the relationships between the activities would be fairly obvious and straight forward as shown below in Figure 4.
However, in reality, using relationships in Primavera P6 is typically more detailed and complex. If you are building a project schedule, and you are not familiar with the activities, or it seems very detailed, you should consult with your project team to ensure you build a schedule that will be considered realistic.
In nearly all branches of Project Management activities have Predecessor/ Successor relationships with one another, as shown below in Figure 5.
Four standard types of relationships can be created between activities in project schedules: Finish to Start, Start to Start, Finish to Finish, and Start to Finish.
Finish to Start (FS)
The FS relationship is one of the most common relationships used in the Primavera P6 schedule. The Preceding activity must Finish before its Successor can start. In our example below, in Fig 6, the walls of the house must Finish being built before the roof installation can Start.
Start to Start (SS)
The SS relationship type is widely used in Primavera P6 scheduling. The Succeeding activity cannot Start until the Preceding activity Starts. This type of relationship is often used to link Start Milestones.
Start Milestones only have a start date in Primavera P6 so an S/S relationship and is commonly used for this purpose. In our example, shown below in Fig 7, our ‘Notice to Proceed’ has a Start Milestone, and ‘Mobilize’ has an S/S/ relationship. ‘Mobilization’ on the project cannot Start until there is a Start instigated by the customer with a ‘Notice to Proceed’
Finish to Finish (FF)
The FF relationship is also widely used in Primavera P6 scheduling. The Succeeding activity cannot Finish until its’ Predecessor activity finishes. This relationship type is often used to link Finish Milestones. In our example below, Fig 8 the ‘Project Complete’ Milestone cannot Finish until the ‘Substantial Completion’ is Finished.
Start to Finish (SF)
The SF relationship is rarely used in Primavera P6 scheduling. The Preceding activity cannot Finish until its’ Successor Starts. Due to the confusion this relationship type causes, and the lack of real-world situations where it can be applied the use of this relationship is actively discouraged in many scheduling guidelines.
In our example, shown below in Fig 8, the ‘Generator’ cannot Finish running until ‘Grid Power’ Starts and is available on site.
When you define the relationships between activities in Primavera P6, care should be taken to ensure that the Predecessor and Successor activities are logically tied together to represent the real sequence of work in the project schedule.
FS relationships are widely used by schedulers with many organizations preferring this relationship as it offers schedule transparency and overall schedule quality.
The SS and FF relationships are often tied to Start and Finish Milestones respectively although they can also be used within the project schedule.
The SF relationship should be avoided with many organizations stating within their scheduling guidelines that it should not be used.