The following sequence activities are often done simultaneously. For PMP Certification purposes, there are three main aspects to sequencing activities. You should be familiar with these and the terminology associated with them.
Determining the logical relationships
Determining the type of dependency
Applying leads and lags
Precedence diagramming method
The precedence diagramming method (PDM) is the preferred method of establishing logical relationships. In this method, each activity is represented as a box, connected to other boxes (activities) with an arrow.
Precedence diagramming method (PDM). A technique used for constructing a schedule model in which activities are represented by nodes and are graphically linked by one or more logical relationships to show the sequence in which the activities are to be performed.
Here are the four types of relationships used in PDM:
Finish-to-start (FS): This is the most common type of relationship. The predecessor activity must finish before the successor can begin. For example:
I have to finish gathering requirements before I start the design.
Finish-to-finish (FF): The predecessor activity must finish before the successor activity can finish. A sample scenario is
I have to finish writing the User Instructions before I can finish editing them.
Start-to-start (SS): The predecessor activity must start before the successor activity can start. As an example:
I need to start writing test questions before I can start writing test answers.
Start-to-finish (SF): This is the least common relationship. You will not see this on the exam, but it’s included here to be complete. The successor activity has to start before the predecessor can finish. Consider this scenario:
A new accounting system has to be up and running before the previous system can be shut down.
Dependency determination
Many times, the relationships in the preceding list are mandatory, but other times they are more discretionary. Some relationships are based on external constraints. The nature of the dependency impacts the flexibility you have in your schedule. Take a look at the different types of dependencies.
Mandatory: These dependencies are required based on the nature of the work involved. For example, you can’t test something not yet developed. Note that mandatory doesn’t refer to a policy; instead, it’s based solely on the physical and logical limitations of the relationship. Using the example of a childcare center, a mandatory dependency would be the rough electrical work that has to be done before the finish electrical.
Discretionary: These relationships are determined by a best practice or resource availability. A best practice example is “I recommend gathering all the requirements before designing the solution, but you can do some design without having the requirements complete and finalized.”
Using the childcare center example, a resource availability discretionary dependency might be planting trees and plants. If the landscaper has one resource that will do both, he can do either task first. The relationship is discretionary and based on his choice.
External: An external dependency is based on something or someone outside the project. Requiring a permit, waiting on a vendor delivery, needing a deliverable from another project, or meeting a regulatory requirement are all examples of external dependencies. The building occupancy permit for the childcare center example is an external dependency.
You can see that mandatory and external dependencies are constraints on how you schedule a project. Discretionary dependencies have the greatest flexibility in your schedule. However, you can still find ways to adjust mandatory dependencies by applying leads and lags.
Leads and lags
A lead is a modification to a relationship that allows you to accelerate the timing between two activities. For example, if you have an FS relationship between gathering requirements and designing the childcare center, you could apply a lead to accelerate designing the center.
Lead. The amount of time whereby a successor activity can be advanced with respect to a predecessor activity.
Here’s an example of a lead: In a finish-to-start dependency with a ten-day lead, the successor activity can start ten days before the predecessor activity has finished. A negative lead is equivalent to a positive lag.
In the childcare center example, the lead would sound like this:
“Two weeks before I am done gathering the requirements, I will start designing the center.”
This is represented as
FS – 2w
where
FS is the finish-to-start relationship.
A lead is shown as a minus sign.
2w signifies two weeks.
Abbreviation What It Means FS Finish-to-start FF Finish-to-finish SS Start-to-start SF Start-to-finish m Month w Week d Day + Add time (a lag) – Subtract time (a lead)
Here’s another example. “I have to finish the rough carpentry by April 1, so I need to have the blueprints signed off three weeks before.” That results in FF – 3w, which shows a finish-to-finish relationship with a three-week lead.
The other type of modification to a relationship is a lag, which indicates a delay in the relationship between two activities. As you can likely guess, a lag is shown as a plus sign.
Lag. The amount of time whereby the successor activity can be advanced with respect to a predecessor activity.
Here is an example of a lag: In a finish-to-start dependency with a ten-day lag, the successor activity cannot start until ten days after the predecessor activity has finished.
Say you want to start an activity 10 days after you start the predecessor activity. Here’s how that would look:
SS + 10d
where
SS signifies the start-to-start relationship.
The plus sign is a lag.
10d is ten days.
Here is another example of a lag. “I will start writing offer letters five days after I finish interviewing.” You’d write that as FS + 5d. The offer letters are dependent upon the completion of the interviews, with a five-day lag.