SYSTEMS DEVELOPMENT

Introduction

System analysis and design is a series of processes for analyzing and designing computer-based

information systems. Systems design allows a development team to roughly see what and how

their system will look like. An important result of systems analysis and design is an application

software, that is, software designed to support a specific organizational function or process.

Key Terms

- Critical path is the longest-duration path through the network.

- Activity is a task that must be performed.

- Event is a milestone marking the completion of one or more activities.

- Program Evaluation and Review Technique (PERT) is a network model that allows for

randomness in activity completion times.

- Structured walkthrough - It is a planned review of system by people not involved in its

development effort.

General Systems Theory

1. Systems Concepts

A system is a set of interacting components that work together to accomplish specific goals.

For example, a business is organized to accomplish a set of specific functions. Any situations,

which involve the handling or manipulation of materials or resources of any kind whether human,

financial or informative, may be structured and represented in the form of a system.

1.1 Characteristics of a System

i. Purpose � Systems exist to fulfill some objective or satisfy a need. A system may accomplish

more than one task. The purpose of a system is closely tied to its rationale.

ii. Rationale � This is the justification for a system�s existence.

iii. Efficiency � This is how well a system utilizes its resources, that is, doing things right using

the least amount of resources.

iv. Effectiveness � How well a system fulfills its purpose, assuming that its purpose is the right

one. Involves a system doing the right things.

v. Inputs� Entities that enter the system to produce output or furnish information.

vi. Outputs� Entities that exit from the system either as interfaces or for end-user activities.

They may be used to evaluate system�s efficiency and effectiveness.

vii. Transformation rules � They specify how the input is processed to produce output.

viii. Throughput � Measures the quantity of work a system accomplishes. Does not consider

the quality of the output.

ix. Boundary � Artificially delimits a system for study or discussion purposes. System

designers can only control those system components within the boundary.

x. Environment � That which impacts the system but is outside the system�s boundary. The

system cannot control events in the environment.

xi. Interfaces � Points where two systems meet and share inputs and outputs. Interfaces belong

to the environment although they may be inside the system boundary.

xii. Feedback� Recycles outputs as subsequent inputs, or measures outputs to assess

effectiveness.

Classification of Systems

Each system can be characterized along a wide range of characteristics.

Physical Systems Vs. Abstract Systems

A physical system consists of a set of elements, which are coordinated and operate as a whole

entity to achieve a certain objective. This system may also be called a concrete system.

An abstract system is an orderly arrangement of conceptual items or components.

Simple Systems vs. Complex Systems

A simple system has few components, and the relationship or interaction between elements is

uncomplicated and straightforward.

A complex system has many elements that are highly related and interconnected.

Open Systems vs. Closed Systems

An open system interacts with its environment. It is a system with a feedback mechanism

that promotes the free exchange of information between the system and the external entities.

Organizations are open systems.

A closed system has no interaction with the environment. This is a system that neither transmits

information to the outside world nor receives any information from the outside world. It is mainly a

scientific concept (e.g. physics experiments).

Open Loop Systems vs. Closed Loop Systems

An open-loop system is one which does not act in a controlled manner, that is, there is no

feedback loop, and so it has no measure of performance against standards.

A closed-loop system is one that functions in a controlled manner. Such a system accepts inputs,

works upon them according to some predefined processing rules and produces outputs. Such a

system is controlled via a feedback loop.

Stable/Static Systems vs. Dynamic Systems

A stable system undergoes very little change over time. A dynamic system undergoes rapid and

constant change over time.

Adaptive Systems vs. Non-Adaptive Systems

An adaptive system is able to change in response to changes in the environment. These systems

can also be described as cybernetic or self-organizing systems.

A non-adaptive system is not able to change in response to changes in the environment.

Deterministic Systems vs. Probabilistic Systems

Deterministic systems operate in a predictable manner. For example, thermostats and computer

programs. In probabilistic systems, however, it is not possible to determine the next state of

the system. These systems depend on probability distribution.

Permanent Systems vs. Temporary Systems

A permanent system exists for a relatively long period of time.

A temporary system exists for a relatively short period of time.

Components of Systems

Inputs

These provide the system with what it needs to operate. It may include machines, manpower,

raw materials, money or time.

Processes

Include policies, procedures, and operations that convert inputs into outputs.

Outputs

These are the results of processing and may include information in the right format, conveyed at

the right time and place, to the right person.

Systems Boundary

A system boundary defines the system and distinguishes it from its environment.

Subsystems

A subsystem is a unit within a system that shares some or all of the characteristics of that system.

Subsystems are smaller systems that make up a super-system / supra-system. All systems are

part of larger systems

Components of Systems

Environment System Boundary

Environment

Input Output

Environment Interacting subsystems

Inputs

These provide the system with what it needs to operate. It may include machines,

manpower, raw materials, money or time.

Environment

This is the world surrounding the system, which the system is a subsystem of.

Objectives and Application of Systems Approach

Fast Forward: There are multiple levels of explanation for any complex situation. All may

be true but their usefulness is different.

Objectives and Application of Systems Approach

Features of Systems Theory

1. All systems are composed of inter-related parts or sub-systems and the system can only be

explained as a whole. This is known as holism or synergy. The systems view is that the whole

is more than just some of its parts and those vital interrelationships will be ignored and

misunderstood if the separate parts are studied in isolation.

2. Systems are hierarchical, that is, the parts and sub-systems are made up of other smaller parts.

For example, a payroll system is a subsystem of the Accounting System,which is a sub of the

whole organization. One system is a sub of another.

3. The parts of a system constitute an indissoluble whole so that no part can be altered without

affecting other parts. Many organizational problems arise once this principle is flouted or

ignored. Changes to one department could create untold adverse effects on others - ripple

effects: e.g. changing a procedure in one department could affect others e.g. admissions -

faculty ,type of data captured, process. , etc.

4. The sub-systems should work towards the goals of their higher systems and should not pursue

their own objectives independently. When subsystems pursue their own objectives, a condition

of sub-optimality arises, and with this the falling of the organization is close at hand!

Information systems designers should seek to avoid the sub-optimality problem!

5. Organizational systems contain both hard and soft properties. Hard properties are those that can

be assessed in some objective way e.g. the amount of PAYE tax with tax code, size of product-

quantifiable

Soft properties - constitute individual taste. They cannot be assessed by any objective standard

or measuring process e.g. appearance of a product, suitability of a person for job and any

problem containing a political element.

Importance of Systems Theory:

a) It provides a theoretical framework for study of performance of businesses

b) It stresses the fact that all organizations are made up of subsystems, which must work together

harmoniously in order that goals of the overall system can be achieved.

c) It recognizes the fact that conflicts can arise within a system, and that such conflicts can lead to

sub-optimization and that, ultimately, can even mean that an organization does not achieve its

goals.

d) It allows the individual to recognize that he/she is a subsystem within a larger system, and that

the considerations of systems concept apply to him/her, also.

e) Given the above factors, it is clear that information-producing systems must be designed to

support the goals of the total system, and that this must be borne in mind throughout their

development.

Systems Theory Concepts

- Entropy � This is the tendency towards disorder (chaos) in a system. The more closed a

system is, the greater the entropy.

- Feedback � This is a control mechanism in open systems. Feedback involves measuring the

output of the system, comparing the output with a standard and using any difference to

modify subsequent input to ensure that the outputs conform to the required standards.

- Feed-forward � It means to take steps that make some adjustments to the system in advance

in order to face any expected deviations in future. Feedback monitors the past results whereas

feed-forward deals with future outcomes.

- Functional Decomposition � This involves factoring a system to its constituent subsystems.

The subsystems are also decomposed further into manageable sizes resulting in a hierarchy

structure of a system. Decomposition is used to analyse the existing system, to design and

finally implement a new system.

- Functional cohesion � Involves dividing into subsystems by grouping activities that logically

go together.

- Coupling � Occurs when two systems are highly interrelated.

- Decoupling � This is a process in which the subsystems are given autonomy and

independence. The subsystems operate independently thereby pursuing own objectives and

enhancing flexibility.

- Synergy � The whole is greater than the sum of its parts. At this point the focus is on global

system needs, not local issues. It means that more than one system working together produce

more and better results than each would achieve independently.

- Optimization � It is possible to achieve the best solution.

Elements of Control Include:

- Goal: This is the expected performance, plan or results.

- Sensor: Measures actual performance.

- Comparator: Compares expected results to actual results obtained.

- Effector: Reports deviation and initiates the response which may lead to a redirection of

activity, revision of the expectation or changing the plan.

- Feed-forward � It means to take steps that make some adjustments to the system in advance

in order to face any expected deviations in future. Feedback monitors the past results whereas

feed-forward deals with future outcomes.

- Functional Decomposition � This involves factoring a system to its constituent subsystems.

The subsystems are also decomposed further into manageable sizes resulting in a hierarchy

structure of a system. Decomposition is used to analyse the existing system, to design and

finally implement a new system.

- Functional cohesion � Involves dividing into subsystems by grouping activities that logically

go together.

- Coupling � Occurs when two systems are highly interrelated.

- Decoupling � This is a process in which the subsystems are given autonomy and

independence. The subsystems operate independently thereby pursuing own objectives and

enhancing flexibility.

- Synergy � The whole is greater than the sum of its parts. At this point the focus is on global

system needs, not local issues. It means that more than one system working together produce

more and better results than each would achieve independently.

- Optimization � It is possible to achieve the best solution.

- Sub-optimization � It is an occurrence that occurs when the objectives of one element or

subsystem conflicts with the objectives of the whole system.

- Sub-optimization � It is an occurrence that occurs when the objectives of one element or

subsystem conflicts with the objectives of the whole system.

- Equifinality � Certain results may be achieved with different initial conditions and in

different ways. In open systems, the same final state can be reached from several starting

points, one result can have different causes, or through different methods, there is more than

one way to achieve the objective.

- Goal-seeking � systems attempt to stabilize at a certain point.

- Holism � the analysis of a system is considered from the point of view of the whole system

and not on individual subsystems. Subsystems are studied in the context of the entire system.

1. Object-Oriented Programming (OOP)

This is a revolutionary concept that changed the rules in computer program development OOP

is organized around �objects� rather than �actions,� data rather than logic. Historically, a program

has been viewed as a logical procedure that takes input data, processes it, and produces output data.

The programming challenge was seen as how to write the logic, not how to define the data. OOP

takes the view that what we really care about are the objects we want to manipulate rather than the

logic required to manipulate them. Examples of objects range from human beings (described by

name, address, and so forth) to buildings and floors (whose properties can be described and

managed) down to the little widgets on your computer desktop (such as buttons and scroll bars).

The first step in OOP is to identify all the objects you want to manipulate and how they relate to

each other, an exercise often known as data modeling. Once you�ve identified an object, you

generalize it as a class of objects and define the kind of data it contains and any logic sequences

that can manipulate it. Each distinct logic sequence is known as a method. A real instance of

a class is called an �object� or, in some environments, an �instance of a class.� The object or

class instance is what you run in the computer. Its methods provide computer instructions and

the class object characteristics provide relevant data. You communicate with objects - and they

communicate with each other - with well-defined interfaces called messages. C++ and Java are the

most popular object-oriented languages today. The Java programming language is designed

especially for use in distributed applications on corporate networks and

the Internet.

Companies often commit significant resources to development, acquisition and continued

maintenance of application systems. These systems often control an organization�s assets and

may in themselves be considered an asset that needs to be protected and controlled.

One or more of the following situations will initiate an individual application or project:

i. A new opportunity that relates to a new or existing business process.

ii. A problem that relates to an actual business process.

iii. A new opportunity that will enable the organization to take advantage of technology.

iv. A problem with the current technology.

v. Organizational growth

vi. Merger or acquisition

vii. Revisions in government regulations

System development projects should be initiated using well-defined procedures to communicate

business needs to management. These procedures often require detailed documentation

identifying the need or problem, specifying the desired solution and relating the potential benefits to

the organization.

Aids in System Analysis and Design Include:

Methodologies � Comprehensive, multi-step approaches to systems development that

guide the work and influence the quality of the final product.

Techniques � Particular processes that an analyst will follow to ensure that the work is

well thought-out, complete and comprehensible to others on the project team.

2. Project Management

A project can be defined as a temporary sequence of unique, complex and connected activities

having one goal or purpose and that must be completed by specific time, within budget and

according to specification. It is a planned undertaking that has a beginning and an end and that

produces a predetermined result or product. Every project is constrained by its scope, time goals

and cost goals.

Projects have the following characteristics:

a) Unique purpose � a project is undertaken to fulfill a specific objective

b) Temporary � projects exist for a limited duration of time; often not perpetual

c) Require resources � such as money, manpower and machine resources

d) Should have a primary sponsor � usually an organisation, a department or individual

e) Involves uncertainty � a great deal of the project implementation is unknown the need for

planning and management.

The key competencies that project managers must develop are known as knowledge areas and

include:

- Scope management

- Time management

- Cost management

- Quality management

- Human resources management

- Communications management

- Risk management

- Procurement management and

- Integration management

The project stakeholders are the people involved in or are affected by project activities (including

project sponsor, project team, support staff, customers, users, suppliers and even opponents to

the project)

Tools � Computer programs that make it easy to use and benefit from the techniques

and to faithfully follow the guidelines of the overall development methodology.

To create new systems or to modify existing ones, information systems professionals follow

several steps:

a) Investigation � the process of understanding a problem or opportunity.

study the organization�s present systems and suggest actions to be taken after doing preliminary

investigation

b) Analysis � the process of defining what the system should accomplish.

c) Design � the process of determining how the system will accomplish its purpose.

d) Implementation � involves creating the system and putting it into use.

e) Maintenance � involves monitoring and changing an information system throughout its life.

System analysts use the system analysis and design process to develop new systems. They

study the organization�s present systems and suggest actions to be taken after doing preliminary

investigation

A project life cycle is a collection of project phases, which includes:

1. Concept

2. Development

3. Implementation

4. Close-out

The first two phases relate to project feasibility while the last two phases focus on delivering the

work and are often called project acquisition.

It is important not to confuse project life cycle with product life cycle. The project life cycle applies

to all projects regardless of the products being produced. On the other hand product life cycle

models vary considerably based on the nature of the product. For information systems a systems

development life cycle (SDLC) is used. SDLC is a framework for describing the phases involved in

developing and maintaining information systems.

Measures of Project Success

A project is successful when:

- The resulting information system is acceptable to the customer.

- May need to specifically mention the importance of functionality of the delivered system

- The system is delivered on time

- The system is delivered within budget

The system development process has a minimal impact on ongoing business operations.

Causes of Project Failures

� Failure to establish top-management commitment to the project

� Lack of organization�s commitment to the system development methodology

� Taking shortcuts through or around the system development methodology

� Poor expectations management

� Premature commitment to a fixed budget and schedule

� Poor estimating techniques

� Over-optimism

� Inadequate people management skills

� Failure to adapt to business changes

� Insufficient resources

� Failure to manage the plan

Systems Planning

Involves:

(i) Project identification and selection i.e. high level planning

(ii) Project initiation and planning i.e. low level planning

Project Identification and Selection

(i) Identify potential development projects

Sources of projects include:

� Management and business units

� Managers who want to make a system more efficient or less costly

� Formal planning groups

Projects are identified by:

� Steering committees

� Top management

� User departments

� Development group or senior information systems staff

Top-down identification focuses on global needs of the organization and is usually done

by top management or steering committees. Bottom-up identification is usually done

by business unit or information system group and doesn�t reflect overall goals of the

organization.

(ii) Classify and rank projects

This process is performed by top management, steering committee, business units or

information systems development group. Value chain analysis is often used. This is a

method to analyze an organization�s activities to determine where value is added and

costs are incurred.

(iii) Select projects

This is the process of considering short and long-term projects. Projects most likely to

achieve business objectives are selected. Decision requires consideration of:

� Perceived and real needs

� Potential and ongoing projects

� Current organizational environment

� Existing and available resources

� Evaluation criteria

� Outcomes

Project Initiation and Planning

Project planning and initiation involves:

� Team organization

� Establishing management procedures

� Identifying scope � Scope defines the boundaries of a project � what part of the business is to be

studied, analyzed, designed, constructed, implemented and ultimately improved?

� Identifying alternatives

� Feasibility/risk analysis and strategic assessment Feasibility is the measure of how beneficial or

practical the development of an information system will be to an organization. Feasibility analysis is

the process by which feasibility is measured.

Risk analysis helps understand and manage uncertainty. There is need to assess

probability, assess impact and establish contingency plan.

Estimation � Estimation of resources, such as human effort, time and cost. Estimation is extremely

difficult and (usually) inaccurate.

Cost/benefit analysis

Development costs are one-time costs that will not recur after the project has

been completed e.g. systems development, hardware/software, user training,

site preparation and data conversion.

Operating costs are costs that tend to recur throughout the lifetime of the

system. e.g. expenses on maintenance, data storage, communication sand

software licenses. Such costs can be classified as:

- Fixed costs � occur at regular intervals but at relatively fixed rates

- Variable costs � occur in proportion to some usage factor

Benefits

Tangible benefits are those that can easily be quantified e.g. cost reduction,

error reduction and increased sales

Intangible benefits are those benefits that are difficult or impossible to quantify

e.g. improved planning and control, improved employee morale and improved

decision making

Constraints

- Schedule e.g. project must be completed before a certain set date

- Costs e.g. the system cannot cost more than 1m

- Technology e.g. the system must be online, use MS Access database and run

- on a Windows platform

- Policy e.g. the system must use double-entry accounting

Scheduling � Usually use of Gantt charts and PERT/CPM methods (Performance

Evaluation and Review Technique/ Critical Path Method). The tools are not mutually

exclusive (especially when PERT is based on �activity on node� conventions). That is

why most project management software tools maintain both views simultaneously.

PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method)

A PERT chart is a graphical network model that depicts a project�s tasks and the relationships

between those tasks. It was developed in the late 1950�s to plan and control large weapons

development projects for the US Navy. It is a project management tool used to schedule,

organise, and coordinate tasks within a project. PERT depicts task, duration, and dependency

information.

Critical Path Method (CPM), which was developed for project management in the private sector

at about the same time, has become synonymous with PERT, so that the technique is known by

any variation on the names: PERT, CPM, or CPM/PERT.

CPM

CPM provides the following benefits:

� Provides a graphical view of the project.

� Predicts the time required to complete the project.

� Shows which activities are critical to maintaining the schedule and which are not.

CPM models the activities and events of a project as a network. Activities are depicted as nodes

on the network and events that signify beginning  or ending of activities are depicted as arcs

or lines between the nodes

Steps in CPM Project Planning

1. Specify the individual activities.

2. Determine the sequence of those activities.

3. Draw a network diagram.

4. Estimate the completion time for each activity.

5. Identify the critical path (longest path through the network)

6. Update the CPM diagram as the project progresses.

1. Specify the Individual Activities

From a work breakdown structure, a listing can be made of all the activities in the project.

This listing can be used as the basis for adding sequence and duration information in later

steps.

2. Determine the Sequence of the Activities

Some activities are dependent on the completion of others. A listing of the immediate predecessors

of each activity is useful for constructing the CPM network diagram.

3. Draw the Network Diagram

Once the activities and their sequencing have been defined, the CPM diagram can be drawn.

CPM was originally developed as an activity on node (AON) network, but some project planners

prefer to specify the activities on the arcs.

4. Estimate Activity Completion Time

The time required to complete each activity can be estimated using past experience or the

estimates of knowledgeable persons. CPM is a deterministic model that does not take into account

variation in the completion time; so only one number is used for an activity�s time estimate.

5. Identify the Critical Path

The critical path is the longest-duration path through the network. The significance of the critical

path is that the activities that lie on it cannot be delayed without delaying the project. Because of

its impact on the entire project, critical path analysis is an important aspect of project planning.

The critical path can be identified by determining the following four parameters for each activity:

� EST - earliest start time: the earliest time at which the activity can start given that its

precedent activities must be completed first.

� EFT - earliest finish time, equal to the earliest start time for the activity plus the time required

to complete the activity.

� LFT - latest finish time: the latest time at which the activity can be completed without

delaying the project.

� LST - latest start time, equal to the latest finish time minus the time required to complete the

activity.

Slack is the amount of time that an activity can be delayed past its earliest start or earliest finish

without delaying the project. The slack time for an activity is the time between its earliest and

latest start time, or between its earliest and latest finish time.

The critical path is the path through the project network in which none of the activities have slack,

that is, the path for which EST=LST and EFT=LFT for all activities in the path. A delay in the

critical path delays the project. Similarly, to accelerate the project it is necessary to reduce the total

time required for the activities in the critical path.

Critical Path: BFJ

6. Update CPM Diagram

As the project progresses, the actual task completion times will be known and the network

diagram can be updated to include this information. A new critical path may emerge, and

structural changes may be made in the network if project requirements change.

CPM Limitations

a) CPM was developed for complex but fairly routine project with minimal

uncertainty in the project completion times. For less routine projects, there is more

uncertainty in the completion times, and this uncertainty limits the usefulness of

the deterministic CPM model. An alternative to CPM is the PERT project-planning

model, which allows a range of durations to be specified for each activity.

Complex projects require a series of activities, some of which must be performed

sequentially and others that can be performed in parallel with other activities. This

collection of series and parallel tasks can be modeled as a network.

In 1957, the Critical Path Method (CPM) was developed as a network model for

project management. CPM is a deterministic method that uses a fixed time estimate

for each activity.

b) While CPM is easy to understand and use, it does not consider the time variations

that can have a great impact on the completion time of a complex project.

The Network Diagram

In a project, an activity is a task that must be performed and an event is a milestone marking the

completion of one or more activities. Before an activity can begin, all of its predecessor activities

must be completed. Project network models represent activities and milestones by arcs and

nodes. PERT originally was an activity on arc network, in which the activities were represented

on the lines and milestones on the nodes. Over time, some people began to use PERT as an

activity on node network. For this discussion, we will use the original form of activity on arc.

The PERT chart may have multiple pages with many sub-tasks.

The milestones generally are numbered so that the ending node of an activity has a higher number

than the beginning node. Incrementing the numbers by 10 allows for new ones to be inserted

without modifying the numbering of the entire diagram. The activities in the above diagram are

labeled with letters along with the expected time required to complete the activity.

Steps in the PERT Planning Process

PERT planning involves the following steps:

1. Identify the specific activities and milestones.

2. Determine the proper sequence of the activities.

3. Construct a network diagram.

4. Estimate the time required for each activity.

5. Determine the critical path.

6. Update the PERT chart as the project progresses.

 1. Identify Activities and Milestones

The activities are the tasks required to complete the project. The milestones are the events

marking the beginning and ending of one or more activities. It is helpful to list the tasks in a table

that, in later, steps can be expanded to include information on sequence and duration.

 2. Determine Activity Sequence

This step may be combined with the activity identification step since the activity sequence is

evident for some tasks. Other tasks may require more analysis to determine the exact order in

which they must be performed.

 3. Construct the Network Diagram

Using the activity sequence information, a network diagram can be drawn showing the sequence

of the serial and parallel activities. For the original activity-on-arc model, the activities are depicted

by arrowed lines and milestones are depicted by circles or �bubbles�.

If done manually, several drafts may be required to correctly portray the relationships among

activities. Software packages simplify this step by automatically converting tabular activity

information into a network diagram.

 4. Estimate Activity Times

Weeks are a commonly used unit of time for activity completion, but any consistent unit of time

can be used.

A distinguishing feature of PERT is its ability to deal with uncertainty in activity completion times.

For each activity, the model usually includes three time estimates:

� Optimistic time - generally the shortest time in which the activity can be completed. It is

common practice to specify optimistic times to be three standard deviations from the mean so

that there is approximately a 1% chance that the activity will be completed within the

optimistic time.

� Most likely time - the completion time having the highest probability. Note that this time is

different from the expected time.

� Pessimistic time - the longest time that an activity might require. Three standard deviations

from the mean are commonly used for the pessimistic time.

PERT assumes a beta probability distribution for the time estimates. For a beta distribution, the

expected time for each activity can be approximated using the following weighted average:

Expected time = (Optimistic + 4 x Most likely + Pessimistic) / 6

This expected time may be displayed on the network diagram.

To calculate the variance for each activity completion time, if three standard deviation times were

selected for the optimistic and pessimistic times, then there are six standard deviations between

them, so the variance is given by:

[(Pessimistic - Optimistic) / 6] 2

 5. Determine the Critical Path

The critical path is determined by adding the times for the activities in each sequence and

determining the longest path in the project. The critical path determines the total calendar time

required for the project.

If activities outside the critical path speed up or slow down (within limits), the total project time

does not change. The amount of time that a non-critical path activity can be delayed without

delaying the project is referred to as slack time.

If the critical path is not immediately obvious, it may be helpful to determine the following four

quantities for each activity:

� EST - Earliest Start time

� EFT - Earliest Finish time

� LST - Latest Start time

� LFT - Latest Finish time

These times are calculated using the expected time for the relevant activities. The earliest start

and finish times of each activity are determined by working forward through the network and

determining the earliest time at which an activity can start and finish considering its predecessor

activities. The latest start and finish times are the latest times that an activity can start and finish

without delaying the project. LS and LF are found by working backward through the network.

The difference in the latest and earliest finish of each activity is that activity�s slack. The critical

path is the path through the network in which none of the activities have slack.

The variance in the project completion time can be calculated by summing up the variances in

the completion times of the activities in the critical path. Given this variance, one can calculate

the probability that the project will be completed by a certain date assuming a normal probability

distribution for the critical path. The normal distribution assumption holds if the number of activities

in the path is large enough for the central limit theorem to be applied.

Since the critical path determines the completion date of the project, the project can be accelerated

by adding the resources required to decrease the time for the activities in the critical path. Such a

shortening of the project sometimes is referred to as project crashing.

 6. Update as Project Progresses

Make adjustments in the PERT chart as the project progresses. As the project unfolds, the

estimated times can be replaced with actual times. In cases where there are delays, additional

resources may be needed to stay on schedule and the PERT chart may be modified to reflect

the new situation.

Benefits of PERT

PERT is useful because it provides the following information:

� Expected project completion time.

� Probability of completion before a specified date.

� The critical path activities that directly impact on the completion time.

� The activities that have slack time and that can lend resources to critical pathactivities.

� Activity start and end dates.

Limitations

The following are some of PERT�s weaknesses:

� The activity time estimates are somewhat subjective and depend on individual group

judgment. In cases where there is little experience in performing an activity, the numbers may

be only a guess. In other cases, if the person or group performing the activity estimates the

time, there may be bias in the estimate.

� Even if the activity times are well estimated, PERT assumes a beta distribution for these time

estimates, but the actual distribution may be different.

� Even if the beta distribution assumption holds, PERT assumes that the probability distribution

of the project completion time is the same as that of the critical path. Because other paths can

become the critical path if their associated activities are delayed, PERT consistently

underestimates the expected project completion time

The underestimation of the project completion time due to alternate paths becoming

critical is perhaps the most serious of these issues. To overcome this limitation, Monte

Carlo simulations can be performed on the network to eliminate this optimistic bias in

the expected project completion time.