Critical path method is a way of planning activities that is based on an algorithm. Using this method implies creating a project model including the following elements:
- A list of all activities needed for project completion
- Relationships between these activities
- A period of time needed for every activity (duration)
Knowing these figures and using critical path method, you can determine the longest sequence of activities needed for project completion as well as the earliest and the latest moments of starting and ending each activity that would help avoid delays. You determine the so-called “critical” activities (located on the longest path) and activities with joint time slots (you can move the deadlines, but the project duration will not change).
Critical path method in managing a simple project
Let’s illustrate this idea with a real-life example – hanging a picture on a wall. What is needed for this mini-project? First of all, let’s create a list of tasks.
- Choose a spot on the wall.
- Buy drywall anchors.
- Choose a picture.
- Drill a hole.
- Screw in the anchor.
- Hang the picture.
Thinking about these tasks, we can understand that some of them cannot be started until the previous ones are finished. It means that some tasks depend on the others. Let’s look at this table:
| Name of the task | Depends on |
|---|---|
| Choose a spot on the wall | - |
| Buy drywall anchors | - |
| Choose a picture | - |
| Drill a hole | Choose a spot on the wall |
| Screw in the anchor | Buy drywall anchors |
| Hang the picture | Screw in the anchor |
Tasks ‘drill a hole’, ‘screw in the anchor’ and ‘hang the picture’ form a sequence of tasks that need to be done in a particular order, one after the other. These tasks are called sequential.
In our example these three tasks and the start of the project (‘choose a spot on the wall’) are the most important critical activities that must be done to solve the problem successfully. These activities are located on the critical path of the project. The most important idea lying in the foundation of critical path analysis is that you can’t start particular activities before finishing others. These activities must be performed sequentially, and every preceding stage must be more or less finished before the other starts.
Critical path consists of the longest sequence of activities from the start of the project till its end and each of those activities must be done according to the schedule so that the project can be finished by the deadline. Tasks on the critical path must be carefully managed. If a task on the critical path is not finished on time, you will have to take prompt action so that the duration of the whole project is not disturbed. Otherwise, you will miss the deadline.
Imagine you have a project that will require 200 days to complete. If the first tasks lying on the critical path is finished one day later, the project will be done in 201 days. To avoid that, you will have to try and finish one of the other tasks one day earlier. So, critical path is a set of all tasks determining the project end date.
A project can have several critical paths because different sequences of actions can be performed simultaneously. For example, in our case tasks ‘choose a picture’ and ‘hang the picture’ as well as ‘buy drywall anchors’, ‘screw in the anchor’ and ‘hang the picture’ form alternative sequences that are as important for project completion.
Critical path in project management can include all the important tasks but not necessarily. Sometimes tasks lying on a critical path turn out to be not that important. In other times, there are tasks that are not lying on a critical path but the whole project depends on their successful completion. To determine a critical path, you need to understand what tasks must be done on time. However, other tasks outside the critical path can be as important and require as much attention.
What are resource restrictions?
Traditional schedules built on the critical path method are based on the cause-and-effect relationship. We have already emphasized these relationships in our plan (for example, it’s impossible to drill a hole before choosing a spot on the wall). However, the project might have restricting conditions connected with the amount of available resources and these conditions must also be taken into consideration. These conditions create additional dependencies that are called resource restrictions.
If you work in a team, all the work on a project can be divided between team members. In our case, you can choose a spot on the wall and drill a hole while your friend goes to buy the drywall anchors and another friend chooses a picture. Several tasks will be done simultaneously.
If you are working on the project alone, you will experience resource restriction (you can’t drill a hole and walk to a store at the same time). In this case the critical path will look differently.
Such critical path is called ‘resource critical path’. This method was suggested as an extension of the traditional critical path method as it allows to take into account resources needed for each task. Schedule that considers resources can include delays induced by narrow spots (lack of resources in the needed moment). Because of such delays the path can become longer.
Project time calculation
In managing projects with a critical way there is the longest sequence of activities. Its duration is the shortest time for project completion.
Coming back to our example, let’s suppose you are doing everything on your own. Let’s write down a calculated duration of every task and determine approximate starting time for each task lying on the critical path. Here is what we get.
| Name of the task | Depends on | Time |
|---|---|---|
| Choose a spot on the wall | - | 10.00 |
| Buy drywall anchors | - | 10.30 |
| Choose a picture | - | 11.00 |
| Drill a hole | Choose a spot on the wall | 11:30 |
| Screw in the anchor | Buy drywall anchors | 12.00 |
| Hang the picture | Screw in the anchor | 12:30 |
Adding up the duration of critical task will give us approximate time for the whole project completion. In our case it’s 1 hour and 26 minutes. Add this to the starting point and you will get the earliest moment of project completion (11:26).
Flexibility of critical path
Critical path method was developed for complex but rather predictable projects. In real life, such projects come across rarely. Schedule created according to critical path method is rarely followed precisely. As mentioned above, any delay of a task on the critical path postpones the project finish date respectively. New requirements or new resource restrictions occur quite often.
Let’s imagine your wife and you decided to redesign your bedroom. You are going to perform the following list of tasks:
- Remove old furniture
- Paint the walls
- Bleach the ceiling
- Bring and arrange new furniture
Meanwhile, your wife has another list of tasks:
- Choose new curtains
- Hang new curtains
Tasks connected to curtains form a subproject and can be viewed as not lying on a critical path. Your wife can choose new curtains and hang them at any point before the project ends. So, these tasks can have flexible dates of starting and ending. These tasks are done simultaneously with the main task chain, so we won’t place them on a critical path.
However, if any of the parallel tasks is being done for too long, you will not finish the project on time. It means you will have to watch closely how parallel tasks are done. For instance, if it takes your wife to choose the curtains too long, the whole project will be delayed.
Redesign of the bedroom will not be finished until there are new curtains on the windows which means that the path that we didn’t see as critical in the first place, becomes such. The initial critical path has changed.
To watch non-critical task, the schedule must be constantly updated. Only this way you will know for sure at which stage your project is at the moment and whether everything is going as planned.