Making a Party out of Project Management: Chapter 3 - Figuring Out the Details - Part 4 of 4

               The next day, Abby continued to work on the schedule of setting up the party. Most of the work on this was done, but she needed to make sure everything could get done in time for the party. To do this, Abby would calculate a very important timeline in project management known as the critical path. The critical path is the absolute minimum amount of time required to complete all the work for a project from start to finish, and any delays in the tasks on the critical path will cause a delay in when the project is finished. If you want to finish a project as soon as possible, what are the tasks that all have to happen one right after another with no break in between them?

               To figure out the critical path, Abby needed to review what tasks had dependencies and how long they all took. When she was sequencing tasks the day before to make sure everyone was available to do tasks at the right time, she made a big flowchart of her tasks to help her to visualize the sequence of tasks from start to finish. Using this flowchart, she started to put little numbers in the upper corners of each box that represented how many minutes into the project a task would start and finish. The range between the little numbers represented the number of minutes expected to complete each task [12].

[12] Note that on a real project, these numbers will likely be in hours or days instead of minutes.

Abby beginning to illustrate the forward pass of sequenced project tasks.

In the upper-left corner of the first task, Abby wrote the number 1. The first task would start in the first minute of party setup. It would take 30 minutes to bring décor and games to the pavilion, so she put a 30 in the upper right-hand corner of the task. This task would be executed from minute one through minute thirty. In the next task after that, she put a 31 in the upper-left corner, as that task was scheduled to start on minute thirty-one. She estimated how long the second task would take and added that duration to 31, the number at the start of the second task, subtracted one, and put the resulting number in the upper-right corner of the second task box. So, for a task following the first one that would take three minutes, she had a 31 in the upper-left corner of that task and a 33 in the upper-right corner of the task (31 plus 3, minus one equals 33). Abby’s décor team would spend minutes 31, 32, and 33 inspecting tables, so that’s why a three-minute task starts at the beginning of minute 31 of the schedule and finishes at the end of minute 33. Performing these calculations similarly for all other tasks in Abby’s network diagram, Abby determined the earliest that all tasks could start using a technique known as the forward pass.

Abby completing the forward pass.

After completing the forward pass, Abby started to do the reverse of what she had completed, now performing a procedure called the backwards pass. As you might guess, Abby started at the very end of her sequence of tasks, on the far-right side of the chart. She had the number 108 (minutes) as the largest number in the upper-right corner of all final tasks on the right. This amount of time was the very soonest that all tasks could be finished.

Abby starting the backwards pass.

That same number was put in the lower-right corner of all three final tasks, as shown below. From that, she subtracted the duration of that task to get the number in the lower-left corner. Then, Abby went to the task boxes just before those final tasks. In the lower-right corner of those boxes, she added the same number as in the lower-left of the final task box, subtracting one. Next, she subtracted from that number the durations of all those second to last tasks, adding one, to figure out the number to put in the lower-left corner of those boxes, and so on [13]. For example, the “Set up food at pavilion” task ended at minute 108 and had a 30-minute duration, so the number in the lower-left box of that task would be 108 minus 30 and plus 1, or 79. Then a task just to the left of that, “Make and pack non-meat food”, would need to end at latest on minute 78 (79 minus 1). Since that task had a 45-minute duration, 78 minus 45 and plus 1 would be 34, so the latest that task could start without delaying the project would be minute 34. As shown in the figure just below, the “Make and pack non-meat food” task would not be able to start before minute 16 of work but needed to get started by minute 34 at the latest to get the project completed on time.

[13] Don’t worry, in the project management world, they use computer software to calculate all this quickly. It’s important to understand how the software works, though, so you can make sound evaluations and decisions based on what the software tells you.

Abby completing the backwards pass.

Abby continued this analysis for all the tasks in the network diagram. It was a lot of work and took her a while to complete, but she finally finished it and checked everything over in time for lunch. Once she had finished it, Abby knew which specific series of tasks would take the longest to complete and thus knew which tasks couldn’t be delayed if the project were to finish in the shortest time possible – she had found the critical path! Finally, Abby had a complete picture of what tasks had to get done without any delay at all, and which tasks could wait a little while if needed. She also understood how long of a wait was too long for any task if the party was to be ready on time.

Abby’s completed project network diagram – note the matching forward and backward pass numbers along the red critical path, where the float is always zero.

Abby now knew how much wiggle room, known as slack or float in project management, she had for tasks that weren’t on the critical path and thus didn’t have to be started as soon as possible. While some tasks can be delayed somewhat without delaying completion of the project, there is a limit to that. Once that limit is reached, a further delay in a task that’s not on the critical path can then cause a delay in finishing the project. Abby could now see how long any of the party’s tasks not on the critical path could be delayed before it would delay the completion of the project and cause the party to start late.

Now that the network diagram was complete, Abby could set the schedule for all the tasks to be done in the right sequence. Her friends knew what tasks they needed to do and when, as well as how long it was expected to take. With the scope of work and the work schedule both defined, Abby was ready to dive into the third primary domain of waterfall project management, developing a budget for the project.

Next post in the series: Making a Party out of Project Management: Chapter 4 - Paying for the Party - Part 1 of 2