Creating a Precedence Diagram (continued)
In the following chart, notice that activity 1.1 is the predecessor to activity 1.2. This means that activity 1.1 must be completed before activity 1.2 can start. Following this logic, activity 2.2 cannot start until both activities 1.2 and 2.1 are both completed. Note that there is no precedence relationship between 1.2 and 2.1, so there is no arrow connecting them.
PDM diagrams are frequently confused with PERT charts. You can tell them apart because PERT charts contain multiple time estimates for each node, whereas PDM networks have a single time estimate for each work package. PERT charts also use activity on arrow (AOA), where the arrow represents the activity and the node is the event; for example, the start or finish of an activity.