By Darrell G. Stiffler, PMP
For years, the Project Management Institute (PMI) has been touting the virtues of the Precedence Diagramming Method (PDM) as the method to use in the Critical Path Methodology (CPM) construction of a network diagram. The network diagram uses boxes or rectangles, referred to as nodes, to represent activities, and by connecting the nodes with arrows it illustrates the logical relationships that exist between the nodes. However, the illustration of a network diagram in the Project Management Body of Knowledge (PMBOK) is OK, but the nodes illustrated are poor. In trying to perform the forward pass, backward pass and calculating float the illustration has no value. Because of the PMBOK vacuum of detail and poor illustration of the diagram node, others have used their own style of node for the critical path calculation. There are many different ways to display the node, and un-standardization has allowed the whole subject to become confusing. I propose that PMI publish in the PMBOK a standard node. This would establish a standard and make it easier for all those creating a PDM much easier.
One of the simpler nodes that I have encountered is what I call the 5 box node (see Figure 1.0). This node model is easy to use if you are experienced in crafting network diagrams. This node contains the minimum of data. The drawback of this model is that information must be written outside the node, such as slack and float and if you are transitioning from a many-to-one relationship the transitioning number must be written on the diagram and then erased after the proper value has been transferred.
Another popular node that I have been exposed to is what I call a Vertical 7 box node (see Figure 2.0) The information for the Early Start and Late Start and the Early Finish and Late Finish is entered by either going down or up in the boxes. For example, when you are performing the forward pass, if you are calculating Late Start you enter the Early Start data, add the duration to the Early Start and enter the sum into Late Start, which is the box below the Early Start. Conversely when you are performing the backward pass you enter the data into the Late Finish box then subtract the duration and enter the data in the box above which is the Late Start box. To me this is counter intuitive. When I think in terms of forward pass and backward pass, I don’t think of going up and down. Additionally, when calculating slack and float, there is nowhere on the model to enter the information.
I propose that the PMI and the PMBOK use the model seen in figure 3.0. When performing a forward pass or a backward pass, values are calculated left to right or right to left, following the flow of the node and the diagram. All information necessary to make decisions would be recorded within the node. Students find this model much easier to use. Another reason I suggest this model as the standard is because when a practitioner or student decides that they want to create the network diagram this model is available in Visio, with slight modification (switching the Task ID and Duration labeling).
If you agree with me, that figure 3.0 is the best approach to illustrating a PDM CPM, let the PMI know. PMI is a great institution and listens to those that are involved. Now if we could just get other organizations to follow that philosophy.