Dependencies in project management shows the relationship between project activities. When you plan to schedule activities, you as a project manager would figure out the relationship between these activities in order to figure out the order of the execution of the activities.
In the activity on node (AON) or precedence diagramming method (PDM), we have seen how the dependencies are shown between the activities.
Also we have seen Activity on arrow diagramming method is limited to show only finish to start (FS) dependencies.
If you observe, the words predecessor and successor are relative words. Meaning that when you use one of these words, you need to mention about at least two activities. For example activity A is a predecessor to activity B.
Types of Dependencies in Project Management
There are four types of dependencies in project management. They are
- Finish-to-start (FS)
- Finish-to-finish (FF)
- Start-to-start (SS)
- Start-to-finish (SF)
Now let us look at some of the interesting things, how to we pronounce or understand this concepts or terminology.
In all the above dependencies in project management, there are two letter that we denoted in the short form. For example FS, FF, SS, SF.
In this the first letter always tells about the predecessor activity. And the second letter always tells about the successor activity. Meaning that in FS (Finish-to-start) dependency, we can read and understand this as the Successor activity cannot start until the predecessor finish the activity.
When we are talking about the predecessor activity and successor activity,
- Always the predecessor activity is independent. Meaning that there is no constraint on the predecessor.
- The constraint of dependency is always on the successor activity.
For all the four dependencies in project management, we can read the statement with the below template statement.
Successor activity cannot “finish/start” until the predecessor activity “finish/start”.
Let us start using the above template for all four dependencies in project management.
Finish-to-start (FS) – Successor activity cannot start until the predecessor activity finish.
Finish-to-finish (FF) – Successor activity cannot finish until the predecessor activity finish.
Start-to-start (SS) – Successor activity cannot start until the predecessor activity start.
Start-to-finish (SF) – Successor activity cannot finish until the predecessor activity start.
Now let us look at each of these dependencies in project management in detail one after the other.
Finish To Start (FS)
Finish-to-start (FS) is the most simplest one among the dependencies in project management. This is the most frequently used dependency when sorting the order of activities in the project.
PMBOK guide defines the finish to start dependency as a logical relationship in which a successor activity cannot start until a predecessor activity finished.
For example in the waterfall SDLC model, the development activity (successor) cannot start until the design finish (predecessor activity).
PMBOK guide defines the finish to finish dependency as a logical relationship in which a successor cannot finish until the predecessor activity has finished.
Meaning that even the successor activity B finishes early it has to wait for activity A to finish first and then only the activity B can finish.
For example, the broadcasting activity cannot finish until the tournament finishes. In this case tournament is the predecessor activity and the broadcasting is the successor activity.
PMBOK guide defines the start to start dependency as a logical relationship in which a successor cannot start until the predecessor activity has started.
So the successor activity B cannot start independently on its own. It has to wait for activity A to start first and then only the activity B can start.
For example, Sales team is going to sell the product after at least documenting the necessary sales information on the product. In this scenario, start to start dependency is stated as follows.
The sales team can’t start selling the product until the basic product road map documentation starts.
This is the rarest one among the other dependencies in project management. And hence it is very difficult to visualize this with an example.
PMBOK guide defines the start to finish dependency as a logical relationship in which a successor cannot finish until the predecessor activity has started.
The successor activity B cannot finish until the start of the predecessor activity A.
PMBOK has an example for this as, the first security guard shift (successor) cannot finish until the second security guard shift (predecessor) starts.
Nature of Dependencies in Project Management
We have seen four types of dependencies in project management. Now depending on nature of the dependency it can be classified into the following.
- External dependencies
- Discretionary dependencies
- Mandatory dependencies
- Internal dependencies
In the project, sometime we may have depend on the activity beyond our work. These are generally known as prerequisites for the project, but still they are dependencies for activities in the project.
For example, you see that generally to perform any installation, you may be dependent on the hardware and software procurement, which may be procured from an external party.
These dependencies are not really the barriers. Instead these dependencies are a matter of preference. Meaning that they are good to fulfill as a matter of preference.
As the name says these dependencies are mandatory and must fulfill at any cost. For example until you download the software, you can install the software on the server.
These dependencies usually are within the project team’s control and hence the name internal dependencies.
Lead and Lag
Let us start with lag, as we all know that lag just means delay.
As per the PMBOK guide,
- A lag is the amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity. Sometimes after completing the predecessor activity, you may have to wait for the successor activity to start in a finish to start dependency. This is nothing but a lag.
- On the other hand a lead is the amount of time whereby a successor activity can be advanced with respect to a predecessor activity. This could be because you wanted to apply some of the schedule compression techniques for example.
To summarize we have started with the dependencies in project management. Then we have seen 4 types of dependencies in project management. They are finish to start, start to start, finish to finish, start to finish.
Also we have seen that depending on the nature of dependencies they categorize in to external, internal, discretionary and mandatory dependencies.
Finally we have conclude the post by looking at what a lead is and what a lag is?
These concepts are extremely important from the PMP exam perspective and also in general in planning and scheduling the project activities as part of project management.