NETWORK SCHEDULING TECHNIQUES
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
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
Since World War II, scheduling techniques have taken on
paramount importance. The most
common of these techniques are shown below:
- Gantt or bar
- Line of balance
Evaluation and Review Technique (PERT)
- Arrow Diagram
Method (ADM) [Sometimes called the Critical Path Method (CPM)]
Diagram Method (PDM)
Evaluation and Review Technique (GERT)
28.2 Benefits and Advantages of Scheduling:
Evaluation and Review Technique (PERT)
perhaps is the best known of all the
relatively new techniques. PERT has several distinguishing
• It forms the
basis for all planning and predicting and provides management with the ability
• 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
a) How time delays in certain elements influence program
b) Where slack exists between elements?
c) What elements are crucial to meet the completion date?
- 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
- 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
The basic requirements of PERT/time as established by the Navy,
in the early 1960s, were as
• 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
• 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
28.3.1 Advantages of PERT:
1. 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
by other planning methods. The technique therefore determines
greatest effort should be made for a project to stay on
2. By using PERT one can determine the probability of meeting
deadlines by development of alternative plans. If the decision
statistically sophisticated, he can examine the standard
deviations and the
probability of accomplishment data. If there exists a minimum of
one may use the single-time approach, of course, while retaining
of network analysis.
3. 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
from the less critical activities to the activities identified
bottlenecks. Other resources and performance trade-offs may also
4. PERT can also evaluate the effect of a deviation in the
actual time required for
an activity from what had been predicted.
5. Lastly, PERT allows a large amount of sophisticated data to
be presented in a
well-organized diagram from which both contractor and customer
Unfortunately, PERT is not without its disadvantages. The
complexity of PERT adds to
the implementation problems. There exist more data requirements
for a PERT -
organized MCCS reporting system than for most others. PERT,
therefore, becomes an
item that is expensive to maintain and is utilized most often on
Many companies have taken a hard look at the usefulness of PERT
on small projects in
recent years. The literature contains many diversified
approaches toward applying
PERT to other than large and complex programs. The result has
been the PERT/LOB
procedures, which, when applied properly, can do the following
- Cut project
costs and reduce time scale
- Coordinate and
- Eliminate idle
- Provide better
scheduling and control of subcontractor activities
- Develop better
- Cut the time
required for routine decisions, but allow more time for decision making
Note that even with these advantages, many companies should ask
they actually need PERT. Incorporation of PERT may not be easy,
even if canned
software packages are available. One of the biggest problems
with incorporating PERT
occurred in the 1960s when the Department of Defense requested
that its customers
adopt PERT/cost for relating cost and schedules. This resulted
in the expenditure of
considerable cost and effort on behalf of the contractor to
overcome the numerous costaccounting
problems. Many contractors eventually went to two sets of books;
was for program control (which was in compliance with standard
company cost control
procedures), and a second set was created for customer
reporting. Therefore, before
accepting a PERT system, management must perform a trade-off
study to determine if
the results are worth the cost.
28.3.2 Criticism of PERT:
The criticism that most people discover when using PERT
- Time and labor
intensive effort is required.
management decision-making ability is reduced.
- There exists a
lack of functional ownership in estimates.
- There exists a
lack of historical data for time–cost estimates.
- The assumption
of unlimited resources may be inappropriate.
- There may exist
the need for too much detail.
28.4 Network Fundamentals and Terminology:
It is important to know that the major discrepancy with Gantt,
milestone, or bubble charts is the
inability to show the interdependencies between events and
activities. These interdependencies
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
The following management information can be obtained from such a
Interdependencies of activities
- Impact of late
- Impact of early
between resources and time
- "What if"
- Cost of a crash
- Slippages in
- Evaluation of
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).
Standard PERT Nomenclature
PERT Sources (Burst Points) and Sinks
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
26. 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 28.3
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.
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
Sequence of Events
The next step is to define the sequence of events, as shown in
Table 28.1 above, which relates
each event to its immediate predecessor. Large projects can
easily be converted into PERT
networks once the following questions are answered:
- What job
immediately precedes this job?
- What job
immediately follows this job?
- What jobs can
be run concurrently?
A typical PERT network is shown in the following figure 28.4.
Simplified PERT Network
The bold line represents the critical path, which is established
by the longest time span through
the total system of events. The critical path is composed of
events 1–2–3–5–6–7–8–9. The
critical path is vital for successful control of the project
because it tells management two things:
1. Because there is no slack time in any of the events on this
path, any slippage will cause
a corresponding slippage in the end date of the program unless
this slippage can be
recovered during any of the downstream events (on the critical
2. 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
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
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 (i.e., Hard Logic):
These are dependencies that cannot change, such as erecting the
walls of a house before
putting up the roof.
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.
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
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.
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
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