6.3 Identifying the critical path
The critical path describes the sequence of tasks that would enable the project to be completed in the shortest possible time. It is based on the idea that some tasks must be completed before others can begin. A critical path diagram is a useful tool for scheduling the dependencies and controlling a project. In order to identify the critical path the length of time that each task will take must be calculated.
We will use the logic diagram for production of the directory of services for carers as a starting point, because it is usual to complete a logic diagram before making a critical path analysis. The length of time in weeks for each key stage is estimated:
|Key stage||Estimated time in weeks|
|B||Negotiate with other agencies||4|
|C||Form advisory group||4|
|D||Establish data collection plan||6|
|F||Write directory text||4|
|H||Agree print contract||2|
|J||Agree distribution plan||12|
We have given the key stage ‘secure funds’ an estimated time of zero weeks because the project cannot start without the availability of some funding, although estimates would provide detail at a later stage. The stages can now be lined up to produce a diagram (see Figure 8: Critical path for directory production) that shows that there are three paths from start to finish and that the lines making up each path have a minimum duration.
If we now trace each of the possible paths to the ‘Finish’ point, taking dependencies into account, the route that has the longest duration is known as the critical path. This is the minimum time in which it is going to be possible to complete the project.
In this example the critical path is A–B–C–D–E–F–I–L, and the earliest completion date for the project is the sum of the estimated times for all the stages on the critical path – 28 weeks – from the point of securing the funding. All the key stages on the critical path must be completed on time if the project is to be finished on schedule.
If the projected total time is a long way off the project sponsor's expectations, you will need to renegotiate the time-scale. Mapping the critical path helps to identify the activities that need to be monitored most closely.
We have used a relatively straightforward example to illustrate this technique, but a more complex project may have several lines of key stages operating at first in parallel but later converging, so that several critical activities may have to be completed before another can start.
A key event list is a statement of identifiable key points along the path of the project together with their scheduled dates. Such a listing may be useful as a concise guide for senior managers. A Key Events list should always be derived from a more detailed plan, preferably critical path based, to ensure that the dates stated are achievable. (Note: the terminology of ‘milestones’ is sometimes used for, and/or confused with, key events.)
|Project brief agreed||project start|
|Design new system||completed by 2nd July|
|Project review (when most of initial work is done)||19th July|
Look at Figure 9 which illustrates the Gantt chart for a project to convert a room into a computer suite. From this chart:
identify the earliest possible time for completion of the project
identify the activities that comprise the critical path
could the project be completed in less time?
The extreme right-hand side of the Gantt chart shows the completion of the final task and so the completion of the project on the 54th day. If you looked back along the chart from that final point you might have identified the critical path as the tasks involving the new floor. The décor had to be specified before floor materials could be ordered and installed and this had to be done before the final decoration of the room.
You might have considered that the project could be speeded up if the ordering time for the flooring could have been reduced – but there were a number of other activities taking place during that waiting time. Some of these other activities also have dependencies, so it is not a simple matter to attempt to reduce the time-scale of a project by speeding up only one activity.
Now compare the Gantt chart in Figure 9 – the chart to show the main tasks needed to convert a room into a computer suite with the critical path network diagram for the same project in Figure 10 – the critical path network diagram of the project to convert a room into a computer suite. Note that the numbers written on the arrows represent the duration of an activity and the flow is from left to right. The numbers in circles are the sequence of tasks in each activity and each task on a line has to be completed before the next one begins. The dotted lines show other constraints. For example, the air conditioning cannot be commissioned (task 23) until the air conditioning system has been designed, ordered and installed, but it also needs electricity (task 21) to work. Similarly, the electricity must be working before the fire prevention system can be commissioned (task 22).
The critical path is the path of longest duration through the network and it is the sequence of tasks involving the new floor. However, there are seven other sequences of tasks that must be completed before task 24, which must be completed before the decoration begins. If there are delays in any of these other activities there may be implications for the critical path and the duration of the whole project.
Although it is essential to identify dependencies, it is very helpful to establish that these are unavoidable. If one activity is usually completed before another it is not necessarily essential to complete it first and it might be possible to overlap the activities. It is an advantage to reduce the number of dependencies because that will increase the flexibility available in implementing the project.
If a task on the critical path is expected to finish five days early, will the project complete five days early?
Make a note of your thoughts.
The project would not necessarily finish five days early because there might be another task that was not critical because it would have finished two days before this unexpectedly early one. In this case, the other task now becomes the critical one and defines the expected finishing time which would now be three days early.