The project logic is the development of the activity sequence or determining the order in which the activities will be completed. The process for developing the project logic involves identifying the predecessors—activities that come before—and successors—the activities that come after.
Project Logic for John’s Move
In our example of John’s move, contacting Dion and Carlita—activity 1.1—comes before the lunch meeting is scheduled. You must logically contact Dion and Carlita before you schedule your Host Planning Lunch—activity 1.2. Your conversation with Dion and Carlita will provide you with dates they are available and establish their commitment to help you move. Therefore, the conversation with Dion and Carlita is a predecessor to the Host Planning Lunch Activity. This relationship is diagrammed in Figure 8.4.
These terms define a relationship that is similar to a family relationship like father and son. The father exists in time before the son. Similarly, each element of the diagram can have predecessor-successor relationships with other elements, just like a father can be the son of someone else. Unlike the biological father-son relationship, activities can have more than one predecessor.
The relationship between a predecessor activity and a successor activity is called a dependency. The successor activity starts after and is dependent on the predecessor activity. Because the conversation with Dion and Carlita must take place before a planning meeting can be scheduled, this is called a natural dependency because the relationship can be inferred logically. Activities that have predecessor-successor relationships occur sequentially—one after the other. Another term for this type of relationship is finish-start, which means the first activity must finish before the next one can start. Refer to Figure 8.4.
Some activities take place concurrently—at the same time. If they must begin at the same time, they have a start-start relationship. If the activities can start at different times but they must finish at the same time, they have a finish-finish relationship. Refer to Figure 8.5.
Predecessor Relationships in John’s Move
Figure 8.6 shows the activities in John’s move with the predecessors identified for the Plan Move and Prepacking groups of activities. Because the finish-start relationship is by far the most common, the type of relationship is assumed to be finish-start unless otherwise mentioned.
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