2 Tasks – Project Dependencies Explained
Part of the process of building a project schedule involves breaking down the work into smaller activities (the Work Breakdown Structure) and then sequencing the activities. When you sequence the activities you should make sure that every activity is related to at least one other activity. In many cases, the relationships will involve two or more activities.
There are a couple of ways to represent these relationships. Perhaps the most common technique is called Precedence Diagramming Method (PDM). (This technique is sometimes called Activity on Node (AON).) In the PDM technique, the activities themselves are placed in boxes and the boxes are connected with arrows that show the precedence relationship.
The most common precedence relationship is when one activity cannot start until another activity has finished. In most schedules this is the relationship that exists in almost all (if not all) cases. This is referred to as a Finish-to-Start relationship. However, there are three other ways that one or more activities can be related to another one. All four are described below.
First, let’s assume we have two activities – “A” and “B”. It does not matter what the exact activities are. It only matters that there is a relationship between them. There are four possible relationships.
This means that Activity B cannot start until Activity A has completed. This is by far the most common relationship between multiple activities. In most schedules, all relationships will be finish-to-start. Example:
- Activity A is “Create the Project Charter.”
- Activity B is “Obtain Project Charter approval from the Project Sponsor.”
- This finish-to-start relationship would say that we must create the Project Charter before we obtain Project Charter approval from the Project Sponsor.
Start-to-finish means that Activity A must start before Activity B can finish. This is a very rare relationship.
Example: Let’s assume that you want to fertilize your garden, but the plants must all be wet when the fertilizer is applied.
- Activity A is to “fertilize the garden.”
- Activity B is to “water the garden.”
- The start-to-finish relationship says we need to start watering the garden (activity B) first to get the plants wet. This activity must continue until the fertilizing starts (activity A). This will ensure the plants remain wet until the fertilizer is ready to be applied. Note that you can start watering at any time and you can finish fertilizing at any time. The relationship only ties the start of activity A to the completion of activity B.
This means Activity A must start before Activity B can start.
Example: Assume that you are having your walls painted in one room and wallpaper is being hung in another room. You want to minimize the total disruption and so you want to make sure both activities happen at the same time.
- Activity A is “Paint the walls.”
- Activity B is “Hang the wallpaper.”
- The wallpaper hangers may be ready to go (activity B). However, the start-to-start relationship says that they cannot start until the painting starts (activity A). This relationship is based on the activity start times. The end times of each activity are not related and, in fact, one activity could end at a much later time than the other.
This means Activity A must finish before Activity B can finish.
Example: Assume you’re cooking dinner and you want the turkey to finish cooking before the potatoes.
- Activity A is “Cook turkey.”
- Activity B is “Cook potatoes.”
- The finish-to-finish relationship says that the turkey must finish cooking (activity A) before the potatoes finish cooking (activity B). This relationship is based on the end times. They can each start whenever they need to, as long as they finish in this order.