Critical Path

2.2.1.7P Critical Path

If you are using a project management scheduling tool, the activities on the critical path are sometimes designated with a red bar for ease of viewing.

(2.2.1.7P.P1)

Critical path refers to the sequence of activities that must be completed on schedule for the entire project to be completed on schedule. If the end-date for the project has slipped, it is because at least one activity on the critical path did not complete on time. It is important to understand the critical path sequence to know where you have schedule flexibility and where you do not. You may have a whole series of activities that end up running late, yet the overall project will still complete on time, since the late activities are off the critical path. On the other hand, if your project is falling behind, placing additional resources on activities that are off the critical path will not result in the overall project completing any earlier.

Note that the critical path will only be calculated if you have sequenced all of the activities in your schedule. The critical path relies on an understanding of the successors and predecessors of each activity. If your activities are not sequenced, the critical path may be calculated erroneously.

The Logic Behind Critical Path (2.2.1.7P.P2)

On every project, no matter how complicated, there are always some activities that can be started earlier or completed later without jeopardizing the final completion date for the project. This flexibility between the earliest time an activity CAN be completed and the latest time when it MUST be completed is called float. There is similar float if the activity has flexibility between the earliest time it CAN start and the latest time it MUST start. By definition, if an activity has flexibility, or float, associated with its start and end-date, then it is NOT on the critical path.

Now let’s look at those activities where you do not have the flexibility in the start and end-dates. These activities cannot be completed earlier because they are pending the completion of another activity. They also cannot be completed later than scheduled without causing all the succeeding activities to be late. That’s because none of the activities that follow have any flexibility, or float, in their start and end-date. All of these activities back up tightly against other activities that precede or succeed them. The critical path consists of the longest sequence of activities that must be started and completed exactly as scheduled. In other words, it is the longest sequence of activities with zero float. If any activity on the critical path is late, the entire project will be late (unless the time can be made up somewhere else on the critical path).

The project end-date is what it is because of the critical path. If there were not a critical path, then there would be at least some float in all the activity paths from start to finish. If there were float everywhere, you could squeeze the float out and finish the project earlier. As you moved the end-date to finish earlier, you would start to remove some of the float. However, at some point, the float would be gone from one of the paths. This would be a point where each activity on the path would have start and end-dates that backed up one against the other. There would be no more float on this sequence of activities. This would be the critical path.

Path B is the critical path since all activities on the path bump up against each other. There is no slack or float in the timeline. The other paths all have some float, which implies that at some point they will wait for other paths to finish. By definition, the critical path has zero float.

For example, say you have a project that is nine months long. Your project management scheduling tool identifies the critical path. Let’s assume that there are 22 activities in the critical path, all of various durations and effort hours. The second activity on the critical path was estimated to be completed in eight days. As the project is proceeding, it turns out that this activity actually took nine days to complete. What you will discover is that now the entire project will take nine months and one day. Delaying the completion of the second activity by one day made the schedule for the entire project go over its deadline by one day. Unless that extra day can be made up somewhere later in the critical path, the project will be completed a day late.

Why is the Critical Path Important? (2.2.1.7P.P3)

Frankly, on many projects it is not necessary to determine the critical path. This is especially true for small projects. However, for larger and more complex projects, it is important to understand the critical path. If the project is trending late and if you are trying to proactively get back on schedule, it is very important to identify the critical path activities. Unless you are able to accelerate activities on the critical path, the end-date for the entire project will remain the same. Applying additional resources to activities that are not on the critical path may allow those activities to be completed early, but they will not affect the overall project end-date. Your chance to make an impact on the projected end-date relies on your ability to identify and shorten the critical path.

The Critical Path May Change (2.2.1.7P.P4)

There are many sequences of activities on a project to get from the beginning to the end. There may, in fact, be multiple critical paths, if they all have no float and all lead to the same end-date. Usually if there are multiple critical paths, they overlap for many of their activities

Given that there are many, many paths through the schedule, it’s possible for the critical path to change. For instance, say you have the same example as above, with 22 activities over nine months. Let’s assume that there is another path of work that includes 19 activities and takes 8 ½ months. If you tried to accelerate the schedule to complete the project in eight months, it gets a little complicated. First you would want to focus on accelerating the activities in the nine–month critical path. However, once the critical path is reduced to 8 ½ months, this second critical path emerges that has the same overall timeline. Compressing the original critical path further will not make the project end earlier, because this second path is still going to take 8 ½ months to complete. In this case, both paths must be accelerated (or perhaps some activities that are common to both can be accelerated).

The other way the path may change is if activities off the critical path get delayed. In the example above, let’s say that one of the activities on the 8 ½ month path ends up taking an extra three weeks. Because there was only two weeks of float in the path, it will now become the critical path and force the entire project to complete one week late.

Calculating the Critical Path (2.2.1.7P.P5)

ES – Early Start

EF – Early Finish

LS – Late Start

LF – Late Finish

There is a manual method for calculating the critical path. This involves looking at the earliest start-date (ES) and earliest finish-dates (EF) for every activity starting at the beginning of a project and moving to the end of the project. You then start at the end of the project and go backward, looking at the latest possible start-dates (LS) and latest finish-dates (LF) for every activity. The difference between the latest start day and the earliest start day for each activity is the activity float (this will end up being the same as the difference between the latest end day and the earliest end day). You then look for the sequence of activities from start to end that have zero float. This is the critical path.

The forward pass involves starting at the first activity in the network diagram and calculating the earliest that every activity can start (ES) and the earliest every activity can finish (EF).

Once the final activity is scheduled using the forward pass, you start at the end and work backward. The backward pass involves calculating the latest that every activity can finish (LF) and the latest every activity can start (LS), while still completing the project on time.

Fortunately, most project management scheduling packages will calculate the critical path for you. All medium to large projects need to use a tool to manage the schedule anyway. Take advantage of this automatic feature if it exists. For a small project, there may only be one major sequence of activities and it should be easy to identify even if you do not use a scheduling tool.

Free Float, Path Float and Total Float (2.2.1.7P.P6)

When you are looking at the float in your project, you may come upon three terms – free float, path float and total float. Free float is a term that is applied to individual activities. It refers to the amount of float in an activity before it delays the next activity. For example, if activity B can start up to five days after activity A completes without impacting the next activity, activity B has a free float of five days.

There is also a concept of path float, which is similar to free float, but is applied over an entire path of the schedule. Path float is shared by all the activities on the path. If one activity uses up some of its free float, it is also reducing the path float that is available for other activities on the path as well. For example, let’s say you have three activities in a path – A, B, C – each with five days of free float. The path float is also 5 days. If activity A starts on time and completes on time, activity B still has the total float of 5 days. However, if activity A is delayed by three days, the path float has now been reduced by three days, leaving a path float of two days remaining. In an extreme example, let’s say activity A takes an extra 5 days to complete. You will now find that activity B and C have no float remaining. Remember that path float belongs to the entire path. If one activity uses the entire path float, there is no float remaining for the other activities on the path. With zero float remaining, this path is now a part of the critical path.

Total float refers to the total amount of float between all activities on all paths. If you have a lot of total float, then you usually have many more options as to how you allocate resources to achieve your due dates and you have more flexibility if your project gets behind schedule. However, if total float on the project is low, you have more schedule risk and much less flexibility. If the project starts to fall behind, you have a harder time reallocating resources, since if another path gets delayed it may quickly turn into the critical path.