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LESSON 28
NETWORK SCHEDULING TECHNIQUES
BROAD CONTENTS
Introduction Benefits and Advantages of Scheduling Historical Evolution of Network Scheduling Network Fundamentals and Terminology Pert/CPM and their Difference Graphical Evaluation and Review Techniques (GERT) Dependencies or Interrelationship Slack Time
28.1 Introduction:
In today’s highly competitive environment, management is continually seeking new and better control techniques to cope with the complexities, masses of data, and tight deadlines that are characteristic of many industries. In addition, management is seeking better methods for presenting technical and cost data to customers.
Since World War II, scheduling techniques have taken on paramount importance. The most common of these techniques are shown below:
- Gantt or bar charts
- Milestone charts
- Line of balance
- Networks o Program Evaluation and Review Technique (PERT) o Arrow Diagram Method (ADM) [Sometimes called the Critical Path Method (CPM)] o Precedence Diagram Method (PDM) o Graphical Evaluation and Review Technique (GERT)
28.2 Benefits and Advantages of Scheduling:
The Program Evaluation and Review Technique (PERT) perhaps is the best known of all the relatively new techniques. PERT has several distinguishing characteristics:
- It forms the basis for all planning and predicting and provides management with the ability to plan.
- It enables management for best possible use of resources to achieve a given goal within time and cost limitations.
- It provides visibility and enables management to control ''one-of-a-kind" programs as opposed to repetitive situations.
- It helps management to handle uncertainties involved by answering the following questions that provides management with a means for evaluating alternatives:
a) How time delays in certain elements influence program completion? b) Where slack exists between elements? c) What elements are crucial to meet the completion date?
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- It provides a basis for obtaining the necessary facts for decision making.
- It utilizes a time network analysis as the basic method to determine manpower, material, and capital requirements as well as providing a means for checking progress.
- It provides the basic structure for reporting information.
- It reveals interdependencies of activities.
- It facilitates "what if" exercises.
- It identifies the longest path or critical paths.
- It allows us to perform scheduling risk analysis.
The above-mentioned benefits apply to all network scheduling techniques, not just PERT.
28.3 Historical Evolution of Networks:
Before going further with the details, let us have an insight into the historical evolution of networks. PERT was originally developed in 1958 and 1959 to meet the needs of the "age of massive engineering" where the techniques of Taylor and Gantt were inapplicable. The Special Projects Office of the U.S. Navy, concerned with performance trends on large military development programs, introduced PERT on its Polaris Weapon System in 1958, after the technique had been developed with the aid of the management consulting firm of Booz, Allen, and Hamilton. Since that time, PERT has spread rapidly throughout almost all industries. At about the same time the Navy was developing PERT, the DuPont Company initiated a similar technique known as the Critical Path Method (CPM) , which also has spread widely, and is particularly concentrated in the construction and process industries.
The basic requirements of PERT/time as established by the Navy, in the early 1960s, were as follows:
- All of the individual tasks to complete a given program must be visualized in a manner clear enough to be put down in a network, which comprises events and activities; that is, follow the work breakdown structure.
- Events and activities must be sequenced on the network under a highly logical set of ground rules that allow the determination of important critical and sub-critical paths. Networks can have up to one hundred or more events, but not less than ten or twenty.
- Time estimates must be made for each activity of the network on a three-way basis. Optimistic, most likely, and pessimistic elapsed-time figures are estimated by the person(s) most familiar with the activity involved.
- Critical path and slack times are computed. The critical path is that sequence of activities and events whose accomplishment will require the greatest expected time.
28.3.1 Advantages of PERT:
- Firstly, a major advantage of PERT is the kind of planning required to a major network. Network development and critical path analysis reveal interdependencies and problem areas that are neither obvious nor well defined by other planning methods. The technique therefore determines where the greatest effort should be made for a project to stay on schedule.
- By using PERT one can determine the probability of meeting specified deadlines by development of alternative plans. If the decision maker is statistically sophisticated, he can examine the standard deviations and the probability of accomplishment data. If there exists a minimum of uncertainty, one may use the single-time approach, of course, while retaining the advantage of network analysis.
- A third advantage is the ability to evaluate the effect of changes in the program.
For example, PERT can evaluate the effect of a contemplated shift of resources docsity.com
must be identified so that a master plan can be developed that provides an up-to-date picture of operations at all times and is easily understood by all.
The interdependencies are shown through the construction of networks. Network analysis can provide valuable information for planning, integration of plans, time studies, scheduling, and resource management. The primary purpose of network planning is to eliminate the need for crisis management by providing a pictorial representation of the total program.
The following management information can be obtained from such a representation:
- Interdependencies of activities
- Project completion time
- Impact of late starts
- Impact of early starts
- Trade-offs between resources and time
- "What if" exercises
- Cost of a crash program
- Slippages in planning/performance
- Evaluation of performance
As we know that networks are composed of events and activities. An event is defined as the starting or ending point for a group of activities, and an activity is the work required to proceed from one event or point in time to another. Figure 28.1 below shows the standard nomenclature for PERT networks. The circles represent events, and arrows represent activities. The numbers in the circles signify the specific events or accomplishments. The number over the arrow specifies the time needed (hours, days, months), to go from event 6 to event 3. The events need not be numbered in any specific order. However, event 6 must take place before event 3 can be completed (or begin).
Figure 28.1: Standard PERT Nomenclature
Figure 28.2: PERT Sources (Burst Points) and Sinks docsity.com
As depicted in Figure 28.2 (a) above, event 26 must take place prior to events 7, 18, and 31. In Figure 28.2 (b), the opposite holds true, and events 7, 18, and 31 must take place prior to event
- Thus, it is similar to "and gates" used in logic diagrams.
However, these charts can be used to develop the PERT network, as shown in Figure 28. below. The bar chart in Figure (A) below can be converted to the milestone chart in Figure (B) below. By then defining the relationship between the events on different bars in the milestone chart, we can construct the PERT chart in Figure (C) below.
Figure 28.3: Conversion from Bar Chart to PERT Chart
Basically PERT is a management planning and control tool. It can be considered as a road map for a particular program or project in which all of the major elements (events) have been completely identified together with their corresponding interrelations. PERT charts are often constructed from back to front because, for many projects, the end date is fixed and the contractor has front-end flexibility.
It is important to note here that one of the purposes of constructing the PERT chart is to determine how much time is needed to complete the project. PERT, therefore, uses time as a common denominator to analyze those elements that directly influence the success of the project, namely, time, cost, and performance. The construction of the network requires two inputs. First, a selection must be made as to whether the events represent the start or the completion of an activity. Event completions are generally preferred.
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- Because the events on this path are the most critical for the success of the project, management must take a hard look at these events in order to improve the total program.
Therefore, by using PERT we can now identify the earliest possible dates on which we can expect an event to occur, or an activity to start or end. There is nothing overly mysterious about this type of calculation, but without a network analysis the information might be hard to obtain.
PERT charts can be managed from either the events or the activities. For levels 1–3 of the Work Breakdown Structure (WBS), the project manager's prime concerns are the milestones, and therefore, the events are of prime importance. For levels 4–6 of the Work Breakdown Structure (WBS), the project manager's concerns are the activities.
28.5 Differences Between PERT and CPM:
Note that the principles that we have discussed so far apply not only to PERT, but to CPM as well. The nomenclature is the same for both, and both techniques are often referred to as arrow diagramming methods, or activity-on-arrow networks. The differences between PERT and CPM are as follows:
- PERT uses three time estimates (optimistic, most likely, and pessimistic). From these estimates, an expected time can be derived. CPM uses one time estimate that represents the normal time (that is, better estimate accuracy with CPM).
- PERT is probabilistic in nature, based on a beta distribution for each activity time and a normal distribution for expected time duration. This allows us to calculate the "risk" in completing a project. CPM is based on a single time estimate and is deterministic in nature.
- Both PERT and CPM permit the use of dummy activities in order to develop the logic.
- PERT is used for Research and Development projects where the risks in calculating time durations have a high variability. CPM is used for construction projects that are resource dependent and based on accurate time estimates.
- PERT is used on those projects, such as Research and Development, where percent complete is almost impossible to determine except at completed milestones. CPM is used for those projects, such as construction, where percent complete can be determined with reasonable accuracy and customer billing can be accomplished based on percent complete.
28.6 Graphical Evaluation And Review Technique (GERT):
Graphical Evaluation and Review Techniques (GERT) are similar to PERT but have the distinct advantages of allowing for looping, branching, and multiple project end results. With PERT one cannot easily show that if a test fails, we may have to repeat the test several more times. With PERT, we cannot show that, based upon the results of a test, we can select one of several different branches to continue the project. These problems are easily overcome using GERT.
28.7 Dependencies or Interrelationships:
There are three basic types of interrelationships or dependencies:
1. Mandatory Dependencies (i.e., Hard Logic): These are dependencies that cannot change, such as erecting the walls of a house before putting up the roof. 2. Discretionary Dependencies (i.e., Soft Logic): These are dependencies that may be at the discretion of the project manager or may simply change from project to project. As an example, one does not need to complete the entire bill of materials prior to beginning procurement.
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3. External Dependencies: These are dependencies that may be beyond the control of the project manager such as having contractors sit on your critical path.
28.7.1 Dummy Activities:
It is sometimes impossible to draw network dependencies without including dummy activities. Dummy activities are artificial activities, represented by a dotted line, and do not consume resources or require time. They are added into the network simply to complete the logic.
In the Figure 28.5 below, the dummy activity is required to show that D is preceded by A and B.
Figure 28.5: Dummy Activity
28.8 Slack Time:
It is essential to know that since there exists only one path through the network that is the longest, the other paths must be either equal in length to or shorter than that path. Therefore, there must exist events and activities that can be completed before the time when they are actually needed. The time differential between the scheduled completion date and the required date to meet critical path is referred to as the slack time. In Figure 28.4, event 4 is not on the crucial path. To go from event 2 to event 5 on the critical path requires seven weeks taking the route 2–3–5. If route 2–4–5 is taken, only four weeks are required. Therefore, event 4, which requires two weeks for completion, should begin anywhere from zero to three weeks after event 2 is complete. During these three weeks, management might find another use for the resources of people, money, equipment, and facilities required to complete event 4.
Therefore, the critical path is vital for resource scheduling and allocation because the project manager, with coordination from the functional manager, can reschedule those events not on the critical path for accomplishment during other time periods when maximum utilization of resources can be achieved, provided that the critical path time is not extended. This type of rescheduling through the use of slack times provides for a better balance of resources throughout the company, and may possibly reduce project costs by eliminating idle or waiting time.
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