In this training video, Devin Deen demonstrates the activity-on-the-node diagramming technique to calculate slack in a project schedule.
Here’s a shot of the whiteboard for your reference!
In Review: How to Find Slack in a Project Schedule
Devin explained the “Activity on the Node” diagramming technique to help highlight risk, refine time estimates and find slack (or float) on your project schedule. This approach provides a visual representation of the project and is a helpful way to detect float or slack.
In this technique, activities are designated as nodes and then broken down into various sectors. You begin by defining the earliest time an activity could begin on a project, the expected time that activity will take, and the earliest time it could finish. Accomplishing this math for all activities, you can detect the earliest possible date the project will finish.
You then repeat this process for the latest date an activity can start as well as the latest date an activity can finish. What you are left with is clear slack in the schedule when you compare those dates, as Devin displays in the chart and diagram of the whiteboard.
Pro-Tip: Only once you’ve determined with the slack is in your project can you then effectively focus the team to get those tasks done, as well as know when to rest a little because you have wiggle room in your schedule.
For more tips on project scheduling, check out Jason Westland’s article: How to Create a Project Management Schedule.
Thanks for watching!
Hi, welcome! I’m Devin Deen, Content Director at ProjectManager.com. In today’s whiteboard session I’m going to teach you to find slack or float in your project schedule.
We’re going to use the activity on the node diagramming technique to do that. In the activity on the node technique. Each of these activities are designated on the node and each of the nodes is broken down into various sectors. So, as the node we of course list the activity in the middle as such, A, B, C, D, E, F, and G. The six remaining sectors we fill in in doing a forward pass through the network and then a backward pass through the network to determine the float.
Let me explain these different areas. In the top left part of the node we’ve got the early start of that particular activity. That’s the earliest time that activity could start going through the project in a forward pass. Then we’ve got our expected time for that particular task and we list the early finish.
The early finish is simply the early start plus the time expected, and you can calculate the earliest finish. Let’s do a node together starting with A. In activity A we’ve got our expected time of 11, so regardless if we’re doing a forward pass or a backward pass it’s still going to have an expected time of duration, so we can fill in both of those two blocks. It’s also going to have an early start. In this case it’s the first task for that project schedule, so it’ll have an early start of zero.
We can then calculate the early finish for that activity, which is zero plus 11. So our earliest finish is 11. Now, we’ll go that successor task to that task, which is C. In task C, on the early start we put 11 because that’s the earliest time that project activity can actually start. We know from our chart, when we’ve worked out with our project team the expected time for C is nine days. So we can again calculate the early finish for activity C, which is quite simply 20.
Now, I’ve got a problem with activity F because I’ve got two predecessor tasks going into it. So how do I determine what is the earliest start for activity F? Quite simply, it’s the largest of the numbers between its two predecessor activities, which in this case is going to be E with 32.
Now I can finish my forward pass through the diagram, 32 and 18, that’s the expected time for task activity F. That’s going to give me a total duration of 50. Now finishing in my finishing node, I’ve got a time of 50 days through this particular path and 33 days. So, really, the earliest time this whole project can be finished is 50 days. Which if we look at it again we can see that A, E, F will take the longest time through our network, which means these items are on the critical path.
Let’s go back and calculate the slack of the schedule by going backwards through the network.
How do we do that? Well, quite simply the earliest time that we can actually finish the project is in 50 days. So, we put a little 50 in the late finish for each of those two nodes at the end. Then, since this is the same here, 50 days early finish, 18 days to finish, so the earliest, sorry the latest that this project node activity can start is 32 days, the same as it was before going forward through the project.
Going backwards, however, you’ll see that since I don’t have to actually start activity G for 37 days, because even if I do it at 37 days starting it it’ll take me 13 days to get through it and I’ll be completing it by the 50th day, which is the same time that I’ll be completing task F. Therefore, I’ve just found that there’s some slack right here along this path. Let’s go back through the diagram going backwards and see where else we can calculate some slack.
Now that I’ve gone backwards through the diagram and calculated all the late finishes and late starts, I can start to look at where my slack is in the schedule. So let’s take the top path, the A, C, G path. We notice that there’s a difference between the early start and the late start, and that different is 17 days. Quite simply, 28 days minus 11 days gives me 17 days. So what that tells me is across this path, I’ve got 17 days of slack.
Again, let’s look at this bottom path. Between paths B, D, F, and then finish I’ve got six days of slack. Quite simply, it’s 13 from 19 days. The early start from the early finish. And so across this bottom path I’ve got six days of slack. This tells me that if I start my task and I follow along my critical chain, I still have six days to play with through this network path through to the end, and I’ve got more days, I’ve got 17 days, to play with on that top end.
It’s important to determine where that slack is so you know where you need to put your effort in focusing the team and getting those tasks done, and when you can back off your effort appropriately because you don’t need to necessarily push them on that path to the end because you’ve got 17 days to play with.
The activity on the node diagramming method is very, very useful, as I said, in helping you determine the risks through the schedule, refining your time estimates, and most importantly helping you find that slack. Which as we know, time is precious to a project manager. For more project management tips and techniques come check us out at ProjectManager.com.