Critical path analysis

Once the network has been produced the next stage is to calculate, starting with the first activity the earliest start and earliest finish of each activity. Programmers usually refer to this calculation process of the earliest times as the 'forward pass' and the latest time as the 'backward pass'. The process of calculating both the earliest and latest activity start and finish is known as time analysis or scheduling. The phraseology used by the various software may be slightly different.

Earliest times

The precedence diagram format used in this example provides for the earliest start date to be entered in the box on the lower left hand side. In figure 10 below the substructure package network is re-produced with the earliest start and earliest finish times calculated on the basis of the logic, durations and lead times previously entered.

Figure 10: Network for simple substructure works - the forward pass

In this illustration the commencement of the project is represented by Activity A. Therefore, this activity has an early start at day 0, the start of the project. It has already been estimated that Activity A has a duration of 10 days and therefore it has an early finish at day 10.

The start of Activity H is constrained by the 'start-to-start' link with Activity A, which has a lead-time of 2 days. Therefore, Activity H can only start at day 2. This activity also has an estimated duration of 10 days and so the earliest it can finish is day 12 (early start of 2 + duration of 10).

This simple process of adding the durations, leads and lags is carried out for all activities and in this substructure package network it can be seen that the earliest that the project can finish is day 30, which is determined by the early finish of Activity E.

In this example for simplicity the times have been indicated by using day numbers. The programming software may indicate these as dates taking account of specified non-working periods such as weekends and selected holidays. The software will generally allow different non-working periods to be applied to the activities. For instance the time required for the curing of concrete may be represented by a discrete activity, or by a lead or lag time and will need to be based on a 7-day week, whereas the works may well be based on a 5-day working week.

Constraints and milestones

Software may also allow the programme logic to be over-ridden by allowing certain constraints to be applied. For example it may be possible to specify that irrespective of the logic an activity can start no earlier than a selected specified date. It is important that only essential constraint dates are added, as they prevent the network from acting dynamically when it is re-scheduled.

Milestones can also be included. These are used to indicate or 'flag' important key events within the project such as building weathertight and 'power on' and are marked on the software by activities that have a duration of zero.

Latest times

Activity E is the final activity for the substructure project. Therefore the earliest date Activity E can finish is the same as the earliest date the overall project can finish, in this case that is on day 30. The latest time at which the project can finish is also day 30 and activity E has a duration of 5 and therefore the latest start is day 25 (latest finish of 30 less duration of 5).

The backward pass is, therefore, the reverse of the forward pass, starting with the last activity durations are deducted rather than added. The resultant analysis is as figure 11 below: 

Figure 11: Network for simple substructure works - the backward pass