What are CPM and PERT?

Lipika Pandey L

A project schedule is a time-oriented and sequenced description of all sorts of project activities. Time is among one of the constraints which every project leader must contend with, viz., scope, schedule, and budget.

There are several techniques for showcasing the project schedule and analyzing the various aspects of the project. Among them, CPM and PERT are the most common techniques used.

What is CPM?

CPM or Critical Path Method is a method based on identifying and managing a path of critical activities which determine the course or duration of the project. A Critical Path is a path used to determine the shortest time to wrap a project. Thus, the CPM theory is based not on the probability but instead on the concept of the preceding task, to determine the duration of the project. This project management technique was identified by the American industry in 1958 as a means of controlling the schedule and cost of the project. In this method, every activity needs to be uniquely defined by its starting and ending points. Earliest Start Time (EST) is the earliest time in which an activity must get started. Apart from EST, there are EFT (Earliest Finish Time), LFT (Late Finish Time), and LST (Late Start Time). In case, any activity starts and ends at the same point in time, then a dummy activity is inserted to identify the two activities.

Some Related Terms

  • Total Float – It is the delay without affecting the project completion. (FT = LFT – EFT)
  • Free Float – It is the positive portion of the total float that can be used by an activity without affecting the total float of the succeeding activity. (FID = Head Slack – Tail Slack)
  • Independent Float – The excess of minimum available time over activity duration is the independent float. 
  • Interfering Float – It is also called head event slack.
CPM VS PERT_ © ProProfs

What is PERT?

PERT or Program Evaluation and Review technique was introduced by US Navy in 1960 to put the worst- and best-case scenarios around the overall project duration. PERT technique was created as a simple risk mitigation approach that can consider the best- and worst-case possibilities of a task estimate. PERT analysis starts with a networked diagram. Each task duration is calculated thrice, the best case, the worst case, and the most likely case, where a best-case schedule is calculated using the best-case estimates, and the worst-case schedule using the worst-case estimate. PERT is determined for each of the tasks involved in the project. In PERT, three values of time are estimated for each activity, viz., optimistic time, most likely time, and pessimistic time.

Te = to+ 4tm+ tp6

Some Related Terms

  • Optimistic Time (to or O) – It is the shortest possible time required to complete the activity.
  • Most Likely Time (tm or M) – It is the time required under normal conditions to complete the activity.
  • Pessimistic Time (tp or P) – It is the longest possible time required to complete the activity. 
  • Standard Deviation (SD) – It is the measure of the amount of variation from the average Mean. 

SD = (P – O)/6

  • Variance – Standard deviation when squared gives variance.

Difference Between CPM and PERT

It is an activity-oriented network.It is an event-oriented network.
The accurate duration of activity can be estimated.Estimates of the duration of activity are not so accurate.
A deterministic concept is used.The probabilistic concept is used.
It can control both time and cost while planning.It is a tool for planning.
Cost optimization is of prime importance.Time is the controlling factor.
Used in construction projects mainly.It is mainly used in research and development projects.

Final Words

While discussing project management CPM and PERT are two important concepts that cannot be missed out. Both have their advantages and disadvantages, but both help to make life easier by managing the projects efficiently. 


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