Critical path analysis

Critical path analysis is a technique that defines the sequence of activities that determine the programme duration, the 'critical path'. As such the critical path can be described as the determination of the shortest possible route from the programme start to finish.

A critical path establishes the relationships between the activities on the programme and applies estimated timings to these activities. The importance of the technique is that it distinguishes between those activities that determine or are critical to the completion date and those that are not. Delays to critical activities will impact on the programme end date; delays to non-critical activities may not.

Critical activities will generally have specific start and finish timings that must be achieved. Non critical activity timings will have an element of 'spare time' which will allow some flexibility in the start and finish timings. This 'spare time' is referred to as float.

Relationships between the project activities

The relationship between project activities must be identified by the programmer in order to establish the timing of the activities and the critical path. There are four basic relationships, or logic links, between project activities: 'finish-to-start', 'start-to-start', 'finish-to-finish' and 'start-to-finish'. These are generally represented in a standard 'precedence diagram format' as follows:

Figure 1: Activity B must follow activity A. Activity A must have finished before activity B can start.

Figure 2: Activity B cannot start until activity A has started.

Figure 3: Activity B cannot finish until activity A has finished.

Figure 4: Activity B cannot finish until activity A has started.

It is important to understand that, in all cases, these relationships are firm and positive. Therefore, in the 'finish-to-start' example, Activity B cannot commence until Activity A is complete.

In general all activities within the programme (other than the first and last) will have at least two logic links; one from an earlier or predecessor activity to the activity start defining the time at which the activity can commence and one linking the activity to later or successor activities.