Calculating Project Slack

Most project managers allow their project management software to calculate the available slack on each activity, but it’s really not that hard to do manually. To find the slack for each activity, you’ll first find the earliest possible start date and the earliest possible finish date for each activity by completing what’s called the “forward pass.” Once you’ve got this info, you do just the reverse through the “backward pass”—you’ll find the latest possible start and latest possible finish date for each activity. There are a few different methods for calculating project slack. Here’s one of the most common approaches.

For this example, we’ll be using a simple network diagram as seen in Figure 7-7. (You can print out Figure 7-7 from the CD if you’d like; it’s in Adobe Acrobat format.) If you examine the network diagram, you’ll see there are two simple paths to completion: ABDF and ACEF. The number over each node represents the duration of the activity. If you add up the duration of each path, you’ll find the critical path— the longest path to completion. In this example, it’s ACEF because it takes 17 days while ABDF only takes 15 days.

Figure 7-7: The longest path to completion is the critical path.

Now let’s try the forward pass. (Again, there are different methods of completing this science, so don’t be alarmed if you’ve been exposed to a different one). Follow these steps:

1. Make the Early Start (ES) for Activity A one because you’ll start on Day one. Add the duration of the activity to the ES and you’ll have three. Now this part trips some folks up: you’ll subtract one day from the value of the ES and the task duration to arrive at the Early Finish (EF) of the activity. The reason is that the duration of Activity A is only two days, not three, right? In other words, if you start on Day one, you should have two days of work to get to Day two. The EF for Activity A is two.

2. The next activities are Activity B and Activity C. The ES for both of these will be three. Why? Because Day three is the next day in the schedule, the earliest possible day to begin either activity.

3. Let’s finish the ES for activities B, D, and F first. The EF for Activity B is the ES, plus the duration, minus one, for an ES of five. The ES for Activity D is six and the EF for Activity D is eight. The ES for Activity F is 9 and the EF for Activity F is 15.

4. Now let’s do activities C, E and F. The ES for Activity C is three and the EF is seven. The ES for Activity E is eight and the EF is ten. Activity F is the last activity in the project so you can bet it’ll be on the critical path—with no slack. The ES for Activity F is actually 11. It’s 11 because Activity F cannot begin until your project team completes Activity E. So, the EF for Activity F is actually 17. Figure 7-8 shows the project updated with all of the ES and EF dates.

   
   Figure 7-8: The ES and EF dates are found by completing the forward pass.

Now that the forward pass has been completed, it’s time to do the backward pass. It’s a cinch; just follow these steps:

1. You’ll begin with the last activity in the Network Diagram, Activity F, which has an EF of 17. You’ll make the Late Finish (LF) the same as the EF value: 17. This is because Day 17 is the latest day the project can finish without being late.

2. The Late Start (LS) for Activity F is the LF value, minus the duration of the activity, plus one. Yes, plus one. Because you’re going backward in the network you’ll add one rather than subtract one. This accounts for the full day of work you have completed on the first activity and the last activity. So, Activity F has an LF value of 17, less the duration of seven, plus one, which equals an LS of 11. It’s no coincidence that the EF and the LF have the same value of 17. It’s also no coincidence that the ES and LS have the same value of 11. It is because this activity is on the critical path.

3. Next let’s do activities D, B, and A. The LF for Activity D is ten—one day prior to the ES of Activity F’s LS. You get the LS for Activity D by subtracting the duration of the activity, plus one, which equals eight. The LF for Activity B is seven and the LS for Activity is five. The LS for Activity A is, well, it’s the first activity in the project. Do you think it’ll have any float? Hey! You’re right—it’s on the critical path so we can skip it for now.

4. Let’s go back and complete the backward pass for E, C, and A. The LF for Activity E is ten and the LS for Activity E is eight. The LF for Activity C is seven while its LS is three. The LF for Activity A is two and it’s LS is one. Figure 7-9 shows the completed backward pass.

   
   Figure 7-9: The backward pass reveals the LF and the LS.

To finalize the process of finding slack, you’ll subtract the LF from the EF and the ES from the LS on each activity. Wherever there’s a zero, you have a task on the critical path; wherever there’s a number, the activity has slack. In this example, activities B and D have two days of slack. Okay, technically they both don’t have two days of slack; there’s two days of slack on the whole project. Or you could say, Activity B and Activity D can each have one day of slack, or either day can have two days of slack. However you slice it, if either activity goes two days beyond its expected completion time this project is late.

Adjusting the Project Schedule

What you will now have to do is adjust and readjust the critical path. This is known as schedule compression. By making adjustments to when tasks begin or by adding additional resources, you can complete the same work in less time.

To begin schedule compression, do the following: