Critical Path: Forward Pass
The highlight of the Project Schedule Management chapter of the Guide to the Project Management Body of Knowledge (PMBOK© Guide) is the critical path method. If you aspire to obtain Project Management Professional (PMP)© status, this scheduling technique has to be mastered.
Before applying the critical path method, we need to know the activities we plan to carry out for this project and what their dependencies are. In other words, we should already have a network diagram that looks like this example:
This is a small project with four activities in it. We have estimated that activities A and B will both take two days, activity C with take three and the final activity, D, will take five. The structure of the diagram shows that activity A has to finish before either activity B or activity C can begin and both activities B and C need to complete before we can get onto activity D.
Now, in order to identify the critical path – i.e. the longest path through the network, or the one with zero float – we need to complete a forward and a backward pass. Anyone who has done a PMP© exam preparation course will remember these. However, the official way of calculating the critical path can be very confusing.
In the following diagram, the forward pass has been completed using the official approach.
This technique implies that the first day of the project is day 1. So, if day 1 is Monday, we will spend Monday and Tuesday carrying out activity A. We expect to finish by close of business Tuesday (day 2). So the Early Start for activity A is Monday (day 1) and the Early Finish is Tuesday (day 2). Now we can get on with activities B and C. They will both start on Wednesday (day 3). Activity B will occupy us during Wednesday and Thursday, finishing on Thursday (day 4). Similarly, we plan to work on activity C on Wednesday, Thursday and Friday (day 5). The 6th working day of the project is likely to be Monday, so that is when we will start activity D. Because activity D depends on both activities B and C, we must wait until they are both complete. Five days’ work is involved in activity D, which will take us up to Friday (day 10).
While the Early Starts and Finishes clearly indicate the days we are starting and finishing, calculating these values requires a bit of thinking. The recommended approach is to add the estimated duration to the activity’s Early Start Date to get the Early Start for the successor activity (or activities). Then we have to subtract 1 from this figure to arrive at the activity’s Early Finish. In the heat of the PMP© exam, the adding and subtracting operations can become confused and it is easy to make a mistake.
Over the years, Velopi’s trainers have evolved a very simple way of calculating the forward and backward passes that leads to the correct answer, but gives us a diagram that is not as useful for assigning work as the official approach, where the first day of the project is day 1.
Our technique is to start at day 0! Then the forward pass involves adding the estimated duration to the Early Start to yield both the Early Finish and the Early Start of the subsequent activity.
You will see that the Early Finish dates are the same as for the official version, but the Early Starts are all one less. This makes sense, because our project starts on day 0 instead of on day 1.
While we acknowledge that the 1-based technique is better if you want to use the schedule in the real world - to assign people to tasks, for instance – the 0-based approach is a great deal more useful inside in a PMP© exam, where you have to answer a question, on average, every 72 seconds.
Of course, converting a 0-based forward pass to a 1-based one simply involves adding 1 day to every Early Start value.
Our next article covers the backward pass.