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These two methods were developed in the 1950s to control large defense projects, and have been used routinely since then. As with PERT chart, Critical Path Analysis (CPA) or the Critical Path Method (CPM) helps you to plan all tasks that must be completed as part of a project. They act as the basis both for preparation of a schedule and of resource planning. During management of a project, they allow you to monitor achievement of project goals. They help you to see where remedial action needs to be taken to get a project back on course.
Within a project it is likely that you will display your final project plan as a Gantt chart (using Microsoft Project or other software for projects of medium complexity or an excel spreadsheet for projects of low complexity). The benefit of using CPA within the planning process is to help you develop and test your plan to ensure that it is robust. Critical Path Analysis formally identifies tasks which must be completed on time for the whole project to be completed on time. It also identifies which tasks can be delayed if resource needs to be reallocated to catch up on missed or overrunning tasks. The disadvantage of CPA, if you use it as the technique by which your project plans are communicated and managed against, is that the relation of tasks to time is not as immediately obvious as with Gantt charts. This can make them more difficult to understand. A further benefit of Critical Path Analysis is that it helps you to identify the minimum length of time needed to complete a project. Where you need to run an accelerated project, it helps you to identify which project steps you should accelerate to complete the project within the available time.
Facts about the critical path:
1. In every project (network), there must exist at least one critical path.
2. More than one critical path may exist. Multiple paths may share some activities.
3. Any critical path must be continuous from the start of the project till its end.
Drawing a Critical Path Analysis Chart
Use the following steps to draw a CPA Chart:
Step 1. List all activities in the plan
For each activity, show the earliest start date, estimated length of time it will take, and whether it is parallel or sequential. If tasks are sequential, show which stage they depend on.
For the project example used here, you will end up with the same task list as explained in the article on Gantt charts (we will use the same example as with Gantt charts to compare the two techniques). The chart is repeated in Table 1.
Table 1: Task List: Planning a custom-written computer project
Step 2. Plot the activities as a circle and arrow diagram
Critical Path Analyses are presented using circle and arrow diagrams.
In these, circles show events within the project, such as the start and finish of tasks. The number shown in the left-hand half of the circle allows you to identify each one easily. Circles are sometimes known as nodes. An arrow running between two event circles shows the activity needed to complete that task. A description of the task is written underneath the arrow. The length of the task is shown above it. By convention, all arrows run left to right. Arrows are also sometimes called arcs. An example of a very simple diagram is shown in Figure 4.
This shows the start event (circle 1), and the completion of the 'High Level Analysis' task (circle 2). The arrow between them shows the activity of carrying out the High Level Analysis. This activity should take 1 week.
Where one activity cannot start until another has been completed, we start the arrow for the dependent activity at the completion event circle of the previous activity.
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