PERT and Project Scheduling With Limited Resources

3.5.7 Project Evaluation and Review Technique (PERT)

Like CPM, PERT is also a network based planning tool developed by US Navy in 1958 and used for scheduling Polaris Missile Project. But, unlike CPM, PERT is used for novel projects like research and development (R & D) where it is difficult to estimate activity duration accurately. CPM is used for projects with prior experience such as civil engineering works.

It is a probabilistic approach for estimating project duration of an activity and event-oriented network diagram. PERT is preferred for those projects in which correct time determination for various activities cannot be made.

PERT uses three-time estimate for each activity with a view to overcoming uncertainty in time estimates.

Optimistic time estimate (t₀): It is minimum time i.e. the shortest possible time required to complete the activity in ideal conditions.

Pessimistic time estimate (t_p): Maximum time required to complete the activity in the worst condition.

Most probable time estimate (t_m): Time required to complete the activity in normal circumstances.

From these three time estimates, we calculate average time i.e. expected time (t_e) using the following formula.

t=(t₀+4t_m+t_p)/6

PERT assumes optimistic time and pessimistic time are equally likely to occur while the most likely time is four times more likely to occur than the other.

Similarly, standard deviation (σ) is calculated using,

S.D. (σ) = (t_p-t₀)/6

And

Variance (σ²) = [(t_p-t₀)/6]²

Difference between CPM and PERT

Similarities between CPM / PERT

Both tools lead to the same end: a Critical Path and Critical Activities with the slack time equal to zero.

Extensions of both PERT and CPM allow the user to manage other resources in addition to time and money, to trade off resources, to analyze different types of schedules, and to balance the use of resources. tensions of both PERT and CPM allow the user to manage other resources in addition to time and money, to trade off resources, to analyze different types of schedules, and to balance the use of resources.

3.5 Project Scheduling with Limited Resources

Planning is usually done for unlimited and readily available resources.

However, in practice, resources are usually limited and scarce. There are many jobs sharing common resources and available resources are not adequate enough.

But, the beauty of the scarce resources is that they can be managed.

To finish a construction project at maximum efficiency of time and budget it requires the sound planning of schedule and allocation of available resources. Resources like man power, materials and equipment are of great importance and requires close attention.

The seasonal shortage, labor disputes, equipment breakdowns, competing demands, delayed deliveries and many other uncertainties affect supply and availability of resources so, this is seldom be taken for granted.

Most project managers are faced with:

• Relatively fixed manpower, a certain number of machines or equipment, and a limited budget.
• Jobs that occur on parallel paths through the network or concurrent activities may require the same resources, and even though precedence constraints would not prevent their being scheduled simultaneously, a limited supply of resources might force them to be scheduled sequentially.
• Lack of adequate resources may lead to the revision of established schedules.
• If the resources demand exceeds the supply, remedial measures to combat inadequate resource supply is to be made. If there are conflicts among project activities for the same resource items, activity duration & precedence relationships should be considered and rescheduling the non- critical activities will often solve the problem.
• In most project situations resources can be acquired or released in practically any desired amounts if one is willing to pay expenses involved in changing resource levels, such as the costs of hiring, training, unemployment insurance, and so on.
• It is usually prudent, however, to maintain relatively stable employment levels and to utilize resources at a more constant rate.
• The scheduler may use activity slack as a means of smoothing peak resource requirements.

3.5.1 Resource Leveling

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It is the method of scheduling activities within their available float so as to minimize fluctuations in a day- to- day resource requirements. It is done by moving project activities. By resource leveling, we try to optimize the use of resources required to complete a project. Resource leveling helps in obtaining uniformity (as far as possible) in resource requirement throughout the life of a project. The benefit of resource leveling is to ease resource management so that cost involved in managing resources can be minimized.

If resource is manpower, its leveling is called “Manpower Leveling”

Usually, activities and their logical sequence are determined, then time duration of each activity is determined. After that only, resources are allocated by Resource Aggregation, Resource Allocation, and Resource Leveling.

Like manpower, Materials schedule is also done based on CPM/PERT analysis.

It helps to deliver materials at the site well in advance but avoids delivery far in advance, as a result of which deterioration, damages etc. are avoided.

Resource leveling is necessary to:

Implement the project effectively.

To reduce the cost of a project.

3.5.2 Resource Smoothing

Resource smoothing is part of resource leveling process. It is defined as a technique that adjusts the activities of a schedule model such that the requirements for resources on the project do not exceed certain predefined resource limits. In itself, resource smoothing is the process that, not withstanding any constraints imposed during the leveling process, attempts to determine resource requirement that is “smooth” and where peaks and troughs are eliminated. Even if there is no limit to the amount of any resource available, it is desirable that resource usage is as smooth as possible.

3.6 Introduction to Planning Software - MS Project

Microsoft Project is a project management software program, developed and sold by Microsoft, that is designed to assist the project manager in developing a plan, assigning resources to tasks, tracking progress, managing the budget, and analyzing workloads.

Bibliography:

IshwarAdhikari and Santosh Kr. Shrestha, “A text book of Project Engineering” 2011, Chandeshwori Publication, First Edition.

K. Nagarajan, “Project Management”, ISBN: 81-224-1340-4, New Age International (P) Limited, New Delhi, India, 2001.

E.R. Yescombe, “Principles of Project Finance” 2002, Yescombe-Consulting Limited.