Program Evaluation and Review Techniques (PERT) Critical Path Method (CPM): A Rough Guide by Andrew Scouller
PROJECT MANAGEMENT Project Managers can use project management software to keep track of the project in real time, and use it as a tool for planning and a guide in case circumstances change. One method which time has proven to be of invaluable use to project managers is the Critical Path Method (CPM). The CPM identifies the relationships between interrelated project components and shows a clear critical path of activities that must be completed by a certain date (project deadline). This helps to create a timescale for the development of the project, allowing the project manager to plan and allocate resources more efficiently e.g. staff. UNDERSTANDING THE SYMBOLS Node this represents the end of the previous activity and the starting point of those activities to follow. à Arrow this shows the direction of the project, as activities follow on one from another. This arrow can only ever point forward on the diagram, since a project cannot go backwards. Square this will contain a number which will represent the earliest start time of this activity. Triangle this will also contain a number which will represent the latest start time of this activity. CALCULATING EARLIEST START DATE 0 A B C D To calculate, for example, the earliest start time of activity C, it is simply the number of days it takes to complete activity A i.e.. However, to calculate this for the final node (the earliest time the project could be finished), the earliest time the project could be finished would be the cumulative of the longest path in the project i.e. Path : B () + D () = Path : A () + C () =
CALCULATING LATEST START DATE 0 0 A B 3 C D To calculate, for example, the latest start time of activity C, it is simply the earliest start time of the previous activity less the number of days that activity takes to complete i.e. - = 3 The latest start time for the final node will always equal the same as the earliest start time. This remains true for the beginning node. THE FLOAT The float of any activity is the number of days the activity could be delayed by but still finish on time before the following activity. It is calculated using this simple formula: - - duration of activity = float So, the float for activities B and D would be 0 as they are on the critical path, but the float for activities A and C would be and repectively; A 3 0 = C = Answer to worked example below: ACTIVITY A B C D E F G H I FLOAT 0 3 0 0 8 0 THE CRITICAL PATH The critical path is essentially where and are equal to each other. The significance of the critical path is that activities on this path cannot have their durations extended, and must start on time so the project is completed in the minimum time allowed. The critical path in essence shows the shortest duration possible for the project. To mark out the critical path, the arrows for the path are often drawn in a different colour, or given double lines as shown above.
A COMPLEX CPM EXAMPLE GANTT CHARTS & STAFFING PROFILES Gantt charts are used to illustrate a project schedule the start and finish dates of each project element. They also show the dependency relationships between activities and can compare the planned and actual progress of the project. Modern systems have a Today line to show the present progress of the project. The length of each activity is plotted on a graph as follows to show the relationship between each project element. *NOTE: there should be a float of days on activity A
Gantt charts can also be used to plan resources and create other charts such as staffing profiles to allocate staff to projects. The numbers in the boxes in this case are the number of staff required to complete each activity. However, there are sometimes limitations to, for example, the number of staff available at a given time, and thus certain activities may not be able to be completed on time. Staffing profiles can help to overcome this particular limitation by moving project elements within their float to satisfy these conditions. Continued example DAY 3 6 7 8 9 0 3 6 Staff Required 3 9 8 8 8 3 3 3 3 3 3 Staff Required 0 9 8 7 6 3 0 3 6 7 8 9 0 3 6 DAY The staffing profile above shows the number of staff required to work on the project each day (black lines). However, if there was a staff restriction of a maximum of 7 staff available on any one day the Gantt chart shows the float of each activity, and allows us to see how activities can be maneuvered to fit this restriction. In this example, move activity H to start on day (see red in above graphs). DAY 3 6 7 8 9 0 3 6 Staff Required 3 9 8 8 8 3 3 3 3 3 3 Restriction 3 7 7 7 7 7 7