If negative, it indicates the amount of time that must be saved so that the project finish date is not delayed. By default and by definition, a task with 0 slack is considered a critical task. If a critical task is delayed, the project finish date is also delayed."
However, there is little distinction between the two; this difference comes from their way of treating the activity time. In PERT, activity time is considered as a random variable, contrary to CPM where each activity requires a single deterministic time value. Furthermore, PERT focuses exclusively on the time variable whereas CPM includes the analysis of the time/cost trade-off.
The task starts from A which requires 7 weeks for completion. Following A, there are two tasks, B and D, which can be carried out simultaneously and they, respectively, require 3 weeks and 8 weeks for completion. Task C immediately follows task B which at least requires 10 weeks (Sum of weeks required by task A and task B) to begin and will take 1 week to finish. Task C is followed by task E which needs both task C and task D to be completed before its start. Since, task C requires 11 weeks and task D requires 15 weeks to finish, therefore, it is logically understood that task E cannot be started before elapse of 15 weeks time-period.
Keeping the above explanation in focus, the earliest start time for each task can be summarized as shown in below table:
Code of task
Earliest Start time
It can be concluded from above analysis that the project will require minimum 22 weeks to complete; in other words, the longest path from start stage till finish stage is minimum duration of the project. This path, called critical path, has been already highlighted with the thickest and darkest line in above figure.
If any task that lies in critical path is delayed, then ultimately the project gets delayed. However, the tasks outside the critical path can be deferred by certain amount of time. For example, task E is not part of critical path, therefore, if it is delayed by 4 weeks, even then so task J will be started as per its earliest start time, as calculated in above table, because task J can only be started once task I, which lies in critical path and requires 21 weeks to finish, is completed.
The above so-far discussion can be summarized in terms of CPM as follows:
Critical Path Method (CPM): = A, D, F, H, I, J
= 7, 8, 1, 3, 2, 1
= 22 weeks
Through CPM, the critical path time can be reduced by carrying out task differently. For example, the time for sanitary and electrical installation (Task D) is 8