(in critical path analysis)
When two paths in an activity network (edges as activities) have a common event but are independent, or partly independent of one another, it is necessary to introduce a dummy activity, essentially a logical constraint, linking the two paths at that point, with the duration of a dummy activity being zero. It is usually shown as a dotted line, and can be labelled di. If R depends on P and Q and S depends on P then the activity network would show:
Activity networks allow at most one activity to be represented by an edge, so if two activities Q, R have to be carried out after one activity P and before another S a dummy activity is required as shown in the figure below.