What is Predecessor and Successor?
A predecessor is an activity whose start or finish controls start or finish of another activity. And a successor is an activity whose start or finish is controlled by start or finish of another activity. In process of determining relationships between activities, predecessor and successor activities are identified first and then they are given a relationship type, for example Finish to Start.
The Concept of Predecessor and Successor:
The concept of predecessor and successor is important to understand. Let us see an example. In the diagram below, there are two activities A and B. Both are linked to each other. Note the originating point of arrow. Arrow is starting from finish of activity A and its ending at start of activity B. Or you can say that start of B is being controlled by finish of A. So A is the predecessor and B is the successor activity and type of relationship between these two activities is Finish to Start. While linking these activities, first step would be to identify predecessor and successor/s. Or you can say which activity is controlling and which activity or activities it controls. Then the relationship type between these activities would be determined, for example finish of activity A controls start of activity B or Finish to Start.
It is often considered that an activity which comes first in schedule is predecessor and activity which comes later is a successor activity, however this is a wrong concept. For example, see the diagram below. Note that both activities have same start. As arrow is originating from start of activity A, this means activity A is a predecessor and its ending in start of activity B so activity B is a successor. This type of relationship is called Start to Start or SS relationship.
Now lets us see another example. In the diagram below, finish of activity B is being controlled by start of activity A. Can you guess the predecessor and successor activity in this case? This type of relationship is called Start to Finish or SF relationship.
So, if you have guessed activity A as the predecessor and activity B as the successor then congratulations, you have understood the concept of predecessor and successor.
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