Project Management has scientific ways of planning, implementing, monitoring & controlling. It considers various parameters of a project. These include money, material, manpower, time, and other resources. It includes Scope, Time & Cost.
We have network planning methods for this. These are PERT and CPM. PERT project management and CPM are two famous managerial techniques. PERT is an abbreviation of the Program Evaluation and Review Technique. CPM is an abbreviation of the Critical Path Method.
PERT was developed by the US Navy for the planning and control of the Polaris missile program. PERT was developed essentially to simplify the planning and scheduling of large and complex projects. It was developed for the U.S, Navy Special Projects Office. This happened in 1957 to support the U.S. Navy's Polaris nuclear submarine project. Emphasis was on completing the program as soon as possible. Later, this found applications across industries. It was used for the 1968 Winter Olympics in Grenoble. They applied PERT from 1965 until the opening of the 1968 Games. This project model was the first of its kind, a revival of scientific management, founded by Frederick Taylor and later refined by Henry Ford. DuPont's critical path method was invented at roughly the same time as PERT.
CPM was developed by DuPont to solve project scheduling problems. Emphasis was on the trade-off between the cost of the project and its overall completion time
Managing a project with these generally involves four steps:
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Program Evaluation and review technique (PERT) is a technique for estimating. It applies a weighted average of optimistic, pessimistic, and most likely estimates when there is uncertainty with the individual task/activity estimates. So, PERT is a method to evaluate and estimate the time required to complete a task. This is like a Three-Point estimate but here we give more weightage to the most likely estimate. The accuracy of single-point task duration estimates may be improved by considering estimation uncertainty and risk. This concept originated with the program evaluation and review technique (PERT). PERT uses three estimates to define an approximate range for a task’s duration:
Most likely estimate (M): This estimate is based on the duration of the task, given the resources likely to be assigned, their productivity, realistic expectations of availability for the activity, dependencies on other participants, and interruptions.
Optimistic (O): The task duration based on analysis of the best-case scenario for the task. This will tell the minimum time the task may take.
Pessimistic (P): The task duration based on analysis of the worst-case scenario for the task. This will tell the maximum time a task can potentially take.
Now let us see how we do estimates using three-point (Triangular Distribution) and PERT.
Let “E” be the estimated time taken for the task after calculation.
The formulas are:
• Three-point estimate / Triangular Distribution: E = (O + M + P) / 3
• PERT: E = (O + 4M + P) / 6
Let us have look and understand what is the critical path method? Critical Path Method (CPM) is one of the Tools & Techniques of Project Time Management. It is used to develop a schedule. Schedule network analysis is a technique that generates the project schedule. It uses analytical techniques. Example: Critical path method, critical chain method, what-if analysis, and resource optimization. These techniques calculate the early and late, start and finish dates, for the remaining portions of project tasks.
Some network paths may have points of path convergence or divergence. These can be seen and used in schedule compression analysis or other analyses.
The critical path method is used to estimate the minimum project duration. This will help determine the amount of scheduling flexibility available on network paths. It calculates the early start, early finish, late start, and late finish dates for all tasks. These do not consider any resource limitations by performing a forward and backward pass analysis through the schedule network, as shown in Figure. We would be using the term task and activity interchangeably.
In this example the longest path includes tasks A, C, and D, and, therefore, the sequence of A-C-D is the critical path. The critical path is the sequence of activities that represents the longest path through a project. This determines the shortest possible project duration. The resulting early and late start and finish dates are not necessarily the project schedule. They show the time periods within which the task could be executed. They use the parameters entered in the schedule model for activity duration, logical relationships, leads, lags, and other known constraints. The critical path method is used to determine the amount of scheduling flexibility on the network paths within the schedule model.
On any network path, the schedule flexibility is measured by the amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint. This is termed as “total float.” A CPM critical path is normally characterized by zero total floats on the critical path. Critical paths may have positive, zero, or negative total float depending on constraints applied. Any activity on the critical path is called a critical path activity. Positive total float is caused when the backward pass is calculated from a schedule constraint that is later than the early finish date that has been calculated during forward pass calculation. Negative total float is caused when a constraint on the late dates is violated by duration and logic. Schedule networks may have multiple near-critical paths. Many software packages allow the user to define the parameters used to determine the critical path(s).
Adjustments to task duration may be required. For example, if more resources or less scope can be arranged, logical relationships (if the relationships were discretionary, to begin with), leads and lags, or other schedule constraints may be necessary to produce network paths with a zero or positive total float. Once the total float for a network path has been calculated, then the free float—the amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint—can also be determined. For example, the free float for Activity B, in Figure, is 5 days
Framework for PERT and CPM:
Task / Activity:
PERT supplies many tools for management with the determination of concepts. Let us have a look at a few
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Now let us look at some advantages and disadvantages of both PERT and CPM
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1. What is PERT and CPM?
Program or Project Management and Review Technique (PERT) is apt for those projects where the time required to accomplish different activities is not known. CPM or Critical Path Method is appropriate for the projects which are reappearing (recurring) in nature.
2. What is a CPM used for?
CPM or Critical Path Method is used to assist the project manager in scheduling the activities. It presumes that the activity time period is known with certainty.
3. When to use PERT and CPM?
PERT is stipulated according to events whilst CPM is ranged towards activities. A deterministic model is used in CPM. While PERT uses a probabilistic model. PERT is generally used in Research and development projects. CPM is generally used in a Construction project, Projects of repeatable nature like residential construction, rollouts, etc.
4. How PERT is Calculated?
PERT is an estimating technique that uses a weighted average of 3 numbers to come up with a final estimate. The following PERT estimate is computed as (O + 4M + P)/6. This is known as “weighted average”.
Both PERT and CPM are managerial techniques that aim at achieving companies' goals, but in the case of PERT, uncertainty component is accepted and states as a part of the system. These can be used in combination with projects too.