Dummies are used in critical path analysis to indicate which activities must be finished before other activities can start. Sometimes it happens that some activities have some preceding activities that need to be finished before they can start. If activities depend on other activities – not all the same activities – then dummies are need to show clearly which activities need to be finished before an activity can start. Dummies are show as dotted lines with an arrow indicating direction.
In the table below C cannot start until A has finished and D cannot start until both A and B have finished.
We need to include a dummy to show D cannot start until B has finished. The dummy is shown below.
Sometimes we need two dummies – or even more – at a time, to show how activities depend on each other.
Dummies are also needed because of what may seem a technical reason. It is not allowed to have two activities between the same two nodes, as shown below.
This is because although two or more activities may depend on the same preceding activities, the finishing of each activity are separate events, so must end at separate nodes. This is only relevant at the terminal node of an activity network, where some of the last activities depend on the same events.
H and G are the last activities. Without the dummy both activities would link to same two nodes. Notice that the length of the dummy is 0, so it is usual to place the dummy between the activity of least duration – G above – and ther terminal node.