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*PRECEDENCE DIAGRAMMING METHOD
The precedence diagramming method (PDM) is 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.
PDM includes four types of dependencies or logical relationships.
A predecessor activity is an activity that logically comes before a dependent activity in a schedule.
A successor activity is a dependent activity that logically comes after another activity in a schedule.
These relationships are defined below and are illustrated in Figure 6.9:
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*Finish-to-start (FS).
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A logical relationship in which a successor activity cannot start until a predecessor activity has finished.
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*Finish-to-finish (FF).
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A logical relationship in which a successor activity cannot finish until a predecessor activity has finished.
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*Start-to-start (SS).
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A logical relationship in which a successor activity cannot start until a predecessor activity has started.
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*Start-to-finish (SF).
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A logical relationship in which a successor activity cannot finish until a predecessor activity has started.
In PDM, FS is the most commonly used type of precedence relationship.
The SF relationship is very rarely used, but is included to present a complete list of the PDM relationship types.
Two activities can have two logical relationships at the same time (for example, SS and FF).
Multiple relationships between the same activities are not recommended, so a decision has to be made to select the relationship with the highest impact.
Closed loops are also not recommended in logical relationships.