Really confused in this. When to use dummy activity exactly? Can someone elaborate with examples. I have been solving a lot of problems related to this and the more I solve new problems, the more confused I get. I have been watching tons of YouTube videos related to this topic and they are getting me even more confused. Please help me here.
Dummy activities may be needed to correctly represent the depndencies between activities
In the following figure, why do we need a dummy activity? Why can't we just connect directly instead of putting a dummy activity?
This one is wrong because C need not wait for D.
This one is wrong because E is not waiting for completion of A.
This one is wrong because there is no activity called O in our activity list.
In order to represent the dependencies between the activities correctly, we need an event 2 that shows completion of A and another event 4 that shows completion of A and D. The only way to do that is to introduce a dummy activity O of zero duration connecting event 2 and event 4. Thus we get the correct diagram:
Part of the confusion is that Dummy Activities are added to make it easier to read (or create) the diagram.
Thus, there are no fixed rules, and different people will put Dummy Activities in different places.
To quote this article that explains it nicely:
Dummy activity forms a connecting link for control purpose or for maintaining the uniqueness of the activity. A dummy activity is used to maintain the network logic and to avoid ambiguity.
Ultimately, the article explain, you want to have as few Dummy Activities as possible:
Too much use of dummy activities in a network creates confusion. Initially, liberal use of dummy activities is followed to fulfill the requirements of inter-relationships between activities. This network is later modified stage-by-stage by removing unnecessary dummies to finally obtain a simple network.
I suggest you read the article, as it provides extensive examples.