2.2.1.3P Precedence Relationships

(2.2.1.3P.P1)

When you are building the network diagram, all of the activities in the work breakdown structure are related to at least two other activities – at least one successor and one predecessor. In many cases, the relationships will involve more than one successor and / or predecessor. There are a couple ways to represent these relationships. 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. This is referred to as a Finish-to-Start relationship. However, there are four ways that one or more activities can be related to each other.

  • Finish-to-Start, 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. For instance (example above), you must install a new computer before the new software can be loaded

  • Start-to-Finish, Activity A must start before Activity B can finish. This is a very rare relationship. For instance (example above), let’s assume that you want to fertilize your garden, but the plants must all be wet when the fertilizer is applied. Therefore, person B must continue to water the garden until person A is ready to fertilize. The relationship says person A must start fertilizing the garden before person B can finish watering the garden. Note that person B can start watering at any time and person A can finish fertilizing at any time. The relationship only ties the start of activity A to the completion of activity B.

  • Start to Start, Activity A must start before Activity B can start. For instance (example above), 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. The relationship says that person A must start painting the walls before person B can start hanging the wallpaper. 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. In this example, it is possible that the wallpaper hangers may be ready to go, but they cannot start until the painting starts. 

  • Finish to Finish, Activity A must finish before Activity B can finish. For instance (example above), assume you are cooking dinner and you want the major foods to complete cooking at approximately the same time. The relationship says that the turkey must finish cooking (A) before the potatoes finish cooking (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.