Critical Path Diagrams
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The critical path method (CPM), and the Project Evaluation and Review Technique (PERT), both devised independently in the 1950’s, but share similarities and now form the basis of many project planning software packages.
The description outline below is simplified approach to CPM and assumes that you can recognize component activities that are required to carry out your project, the sequence(s) in which they must take place and how long each will take.
The purpose of CPM is to permit you to recognise, which activities lay on the ‘critical path’ – i.e. those for which any setback or rushing will affect the overall time for the project. This will assist you in managing the collection of tasks to accomplish fixed time targets overall.
More advance forms of CPM also know about the cost of each element, so overall costs can be managed as well as timing.
The fundamental elements of a critical path diagram (illustrated above) are:
- Arrows (blue) that represent activities – area of work that use up time or resources – e.g. ‘build wall’, ‘train personnel’, ‘print 1000 leaflets’.
- These start (green) and end (red) circles that represent events – points in time that usually mark the start or end of an activity (e.g. ‘start wall’, leaflets arrive’); events do not, themselves, consume time or resources.
- Sometimes you also need dashed arrows that indicate sequence (i.e. where one event must be completed before another starts even though they are not directly linked by an activity).
- List all the activities and sub-activities required to accomplish your project and identify the events that start or end each of these activities.
- Construct the map as illustrated above, showing the overall sequences you require. No event can happen until all the activities feeding into it are complete and no activity can start until the event it follows has happened. Unlike flowchart methods of representing action plans, classic CPM networks have no loops, optional routes or decision nodes. Every activity must happen in the order shown, and once it has happened, it can’t happen again. The diagram is drawn as if you have made all the decisions in advance and know exactly what has to happen, in what order (however see point 4 below!)
- Check the diagram carefully, adding any details needed to make it function correctly.
- Work out the earliest and latest possible start times of each activity, where there is slack, and where the critical path lies. Reviewing the example above, it is clear that they start building the walls on the second day, start tiling the roof on the sixth day, and complete at the end of the seventh day. The sequence of activities that goes through the upper branch is the critical path because any delay anywhere in this sequence adds to the total; there is no slack. However, the bottom branch does have slack in it - it needs only 2.5 days while the top branch needs 4 days.
- Adjust as required should things not go as planned, amending the diagram to meet the new conditions, but these alternative possibilities are in your head; they are not shown on the diagram itself.