How to Use Forward and Backward Pass for Activity Assignments

I’m sure you know that a good schedule is critical for the project success. However, did you know that network diagrams are extremely important for creating a good project schedule?

In my experience, many senior project managers and project planners shun network diagrams. They just don’t see any value in creating or analyzing them. In fact, I have noticed that some experienced project managers don’t even know what a network diagram is.

I have not written this post to explain the basics of a network diagram. Instead, the following article will be useful for you if you already know how to prepare a network diagram and can do forward and backward pass.

The Not-so Popular Network Diagram

There are many reasons for mixed feelings towards network diagrams. They tend to be bulky and difficult to create and maintain. Furthermore, before advancement of software scheduling tools, creation and analysis of network diagrams required a lot of time.

In the past, there were valid reasons for not using network diagrams. However, in the last twenty years or so, scheduling software (like MS Project) has become extremely powerful. You just need to enter project activity data, and viola! your scheduling tool will automatically generate a network diagram for you.

Even so, many project managers still avoid using network diagrams. It could be because of time constraint. Most project managers are under the gun to finish their projects in a limited amount of time, so they avoid (seemingly) unnecessary tasks like creating and analyzing network diagrams and choose instead to create a rudimentary project schedule and jump right into executing their projects.

Lack of enthusiasm for the network diagram could also be due to the popularity of scheduling bar charts (aka Gantt charts). Many project managers believe that the Gantt charts are panacea for all scheduling problems, and, although Gantt charts are good for tracking and reporting, network diagrams are much more useful for scheduling analysis.

I believe, in today’s day and age, there is no excuse for not using network diagrams for project scheduling. Some people choose cheap web based scheduling tools or spreadsheets, which do not even support network diagrams, but even that is not a good enough reason to avoid them. To make a good project schedule, you need to analyze network diagrams, especially when it comes to activity assignments.

Analyzing Network Diagrams

Activities (or tasks) are represented as nodes or boxes in a network diagram. A typical node is shown in the figure below.

Fig. I – Network Diagram Node

Each node in a network diagram has seven elements:

1. Activity (Task) Name
2. Duration (D) – Number of time units required to finish an activity
3. Early Start (ES) – An activity can start as early as this date
4. Early Finish (EF) – An activity can finish as early as this date
5. Late Start (LS) – An activity can start as late as this date without delaying the project
6. Late Finish (LF) – An activity can start as late as this date without delaying the project
7. Slack (Float) – The amount of time that a schedule activity may be delayed from its Early Start date without delaying the project finish date

Let’s take a small scheduling example and analyze its network diagram. The figure below depicts a sample project schedule.

Fig. II – Tabular Project Schedule

The network diagram for our sample project is shown in the figure below.

Fig. III – Project Schedule Network Diagram

We will now use the above schedule for doing activity resource assignments. Let’s assume the following things for further analysis:

1. Only one resource is required for each activity.
2. The resource skills are interchangeable.
3. The tasks are noncontiguous and can be split.
4. The project can use a maximum of 2 resources on any given day.

Figure IV depicts early schedule of the project. It is made using forward pass dates.

The top of the chart shows the resource requirement for each activity on each day of the project. It also highlights total available float in yellow. The bottom portion of the chart shows total requirement, availability, and shortfall of resources on each day.

Fig. IV – Forward Pass Project Schedule

Figure V below depicts late schedule of the project. It is made using backward pass dates.

Fig. V – Backward Pass Project Schedule

Compare the Forward Pass Project Schedule with the Backward Pass Project schedule. You’ll notice that on some days there is a shortfall of resources while on other days there is an excess.

Now let’s consider a resource optimized schedule. The next figure shows a resource levelled schedule where resources are completely utilized.

Fig. VI – Resource Optimized Project Schedule

This schedule was made using activity floats and by adjusting the Start and Finish dates. By doing this, we are able to fully utilize the available resources. This schedule can only be prepared by finding the activity floats and analyzing the network diagram. It should be noted, also, that this type of analysis is not possible just using tabular charts or a Gantt chart.

Gantt charts are complementary to network diagrams, and they too should be used for project scheduling, but they do not provide the whole picture.

Conclusion

I believe network diagrams are extremely important for scheduling analysis, and network diagrams, as well as analysis of network diagrams, is next to impossible without a scheduling tool like MS Project. What do you think? Have you used network diagrams for project scheduling? Do you think they complement scheduling bar charts? Or, do you think project scheduling can be done without network diagrams?

I would love to hear your comments below.