Scheduling Tools: GANTT CHARTS, PERT, CPM

<< Schedule: Scheduling Fundamentals

Risk and Change Management: Risk Management Concepts >>

Software Project Management (CS615)

LECTURE # 38

8. Scheduling

8.3

Scheduling Tools

GANTT CHARTS

There are various tools that help you create a schedule. One of the simplest

project management tools used to represent the timeline of activities is the Gantt

chart.

Long before the advent of computers, Henry L. Gantt lent his name to a simple

and very useful graphical representation of a project development schedule.

The Gantt chart shows almost all of the information contained in the schedule

activity list, but in a much more digestible way. The schedule information is more

easily grasped and understood, and the activities can be easily compared. The

Gantt chart enables us to see at any given time, which activities should be

occurring in the project.

A Gantt chart has horizontal bars plotted on a chart to represent a schedule. In a

Gantt chart, you plot time on the horizontal axis and activities on the vertical axis.

You represent an activity by a horizontal bar on the Gantt chart. The position of a

horizontal bar shows the start and end time of an activity and the length of the bar

show its duration. You can have one look at the Gantt chart and make out the

progress of the project. Figure 1 displays a sample Gantt chart.

290

Software Project Management (CS615)

Activities

PLANNED

Time (Person days)

ACTUAL

Figure 1: Example Gantt Chart

In Figure 1, activity D starts on day 1 of the project. The planned duration of this

activity is 20 days. The planned duration is shown in a lighter shade on the Gantt

chart. This is done to differentiate planned duration from the current status of the

activity. In the case of activity D, the current duration of the activity is 19 days.

Therefore, activity D is still one day short of completion. This can be discerned

from the length of the gray and black bars. Activity C, planned for completion on

day 40, is much behind schedule. This can be observed from the smaller length of

the black bar in the Gantt chart.

To understand how you can use a Gantt chart to schedule a project, consider an

example. Table 1 display a set of activities in a software project and the start and

end time for each activity.

291

Software Project Management (CS615)

Table 1: Project Activities and Time Allocation Details

292

Software Project Management (CS615)

line indicates the status of the project on a specific day. The left side of the line

indicates the tasks that are completed. On-going activities run across the line. The

future activities lie completely to the right of the line. After drawing the Gantt

chart you can extend the bars every week to a length proportional to the work

completed during the week. Therefore, the bars describe the status of the-project

at any point of time.

This process of creating a Gantt chart as discussed above is manual. Nowadays,

automated tools are available for creating Gantt charts. For example, you can use

Microsoft® ProjectTM 2000 developed by Microsoft to create a project schedule

automatically.

Figure 3 is another example of a Gantt chart. The symbols used in the chart are

widely accepted, though not standardized. The inverted triangle, for example, is

commonly used to represent a significant event, such as a major milestone.

The Gantt chart in Fig. 3 demonstrates the ease with which important schedule

information can be quickly perceived. We can immediately see that, except for the

maintenance phase, all phases overlap, and that from November to mid-December

1992 three high level activities overlap.

Major milestones

SRR = Software requirements review

POR = Preliminary design review

COR = Critical design review

TRR = Test readiness review

ATP = Acceptance test procedure

293

Software Project Management (CS615)

SPR

Requirements

analysis

PDR

Top level design

CDR

Detailed design

Implementation

TRR

Integration

ATP

Testing

Maintenance

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr

1992

May Jun

1993

Major milestones

SRR = Software requirements review

PDR = Preliminary design review

CDR = Critical design review

TRR = Test readiness review

ATP = Acceptance test procedure

Figure 3: High level Gantt chart -project development schedule.

More detailed charts can also include the names of the engineers assigned to each

activity, and the equipment that will be needed for each activity. This information

can be added next to the activity time lines in the graph, or as an inserted

reference table (similar to the list of major milestones in fig 10.1). Some

variations of the Gantt chart do include this type of information on the chart, but

this can cause clutter, which is contrary to the main objective of the chart; to

enable important schedule information to be grasped quickly.

It is also important to understand what Gantt charts do not provide. In a Gantt

chart, it is difficult to provide information on the amount of resources required to

complete each activity. A common mistake is to conclude that if five engineers

are assigned to integration, and the integration activity starts in mid-September

1992 and ends in mid-January 1993 (four months), then integration requires 20

work months. In fact, integration may start with only one engineer, with one more

joining during the second month, and the remaining three engineers joining during

the third month. The integration team may then be reduced to three engineers

during the fourth integration month.

Figure 3 includes only seven activities. As more detail becomes available, lower

level activities can be included on the chart. When the chart has more activities

than it can reasonably carry (a subjective decision), additional charts may be

294

Software Project Management (CS615)

added. For example, the design activity can be presented on a separate Gantt chart

(see Fig. 4).

Figure 4 presents both high and low level activities. For example, 'Integrate phase

1 model' contains three low level activities: 'Integrate executive', 'Integrate

operating system' and 'Integrate user interface'. This provides the continuity link

between the detailed Gantt chart (Fig. 4) and the higher level chart (Fig. 3).

Set up

integration site

Integrate phase

I model

Integrate

executive

Integrate

operating

system

Integrate user

interface

Integrate phase

TRR

II model

Sep 15

Oct

Nov

Dec

Jan

1992

1993

Figure 4: Detailed Gantt chart -integration schedule

Note that each period of one month in Fig. 3 has been divided into four weeks.

Though not completely accurate, this is a common approximation, used also in

estimation and apart from being convenient; it also provides some slack for minor

scheduling adjustments.

Similar detailed Gantt charts can be prepared for each of the major project

development phases. Non-development activities will also appear on the Gantt

chart, such as 'Procurement of development tools', or 'Market research'. This is

particularly useful when certain development activities are dependent on other

non-development activities, such as the procurement of development tools (e.g. a

compiler) that need to be completed before the implementation activities can

begin. In cases where such dependent relationships may have been overlooked,

they will often emerge from' a review of the Gantt chart. This type of dependence

295

Software Project Management (CS615)

between activities is best presented in another type of chart, called a Network

precedence chart or a PERT chart.

Network Scheduling Techniques

To plan the activities in a project, you can also use network-scheduling

techniques. Network scheduling techniques use network schedules to trace the

completion of predetermined activities.

There are two basic network-scheduling techniques:

PERT and

CPM

You can use either of these techniques to analyze a wide variety of projects. Each

technique depicts a project as a sequence of activities. This helps you perform an

analysis of individual activities or the complete project.

The network-scheduling techniques also enable you to analyze the dependencies

that exist between the activities.

Using PERT and CPM, individually or in combination, helps you complete a

project on time. By using these techniques, you can determine the latest time by

when an activity should start to be completed on time.

Despite the different approaches followed by PERT and CPM, both techniques

have some common components. These include:

Activities

Nodes

Network

Critical path

a) Activities

Activities are the basic building blocks of network schedules. An activity is

defined as a task that consumes time, effort, money, or any other resource. It is

necessary to specify all the activities of a project by breaking down a project into

several steps.

You need to define the steps in such a way that they are distinct, homogeneous

tasks for which you can estimate resource requirements.

Each activity is represented on a network schedule by using an arrow with its

head indicating the direction in which the project will progress. Each activity is

identified by a description or an alphabet. In addition, the estimated duration of

296

Software Project Management (CS615)

each activity is placed below the activity. Figure 5 shows an activity with

expected duration of 15 days.

15 Days

Figure 5: An Activity

After identifying all the activities in a project, it is necessary to schedule them.

This enables you to arrange the activities in the order of completion. You

sequence the activities based on their types.

The different activities that are possible in a project are predecessor, successor,

and concurrent activities.

Predecessor activities need to end before the next can begin. After a predecessor

activity is completed, the successor activity becomes the predecessor for another

activity. However, unlike the predecessor arid successor activities, the concurrent

activities can be completed simultaneously with other activities.

Some activities in a project, irrespective of whether they are predecessor,

successor, or concurrent, may depict a float period. Float is the amount of time by

which an activity may be delayed without affecting the completion date of the

entire project. However, the complex dependencies that exist between activities

result in sequencing constraints for projects. To a large extent, these constraints

limit the flexibility that you may otherwise have in project planning.

b) Nodes

A node on a network schedule is that point in time at which an activity either

begins or ends. The point where an activity begins is called a tail node and the

point where it end is called its head node. On a network schedule, a circle

represents a node. A number identifies each node in a network schedule. Figure 6

represents the tail and head nodes for the activity of interviewing clients in a

project.

Figure 6: An Activity Connecting Two Nodes

c) Network

297

Software Project Management (CS615)

A network is the graphic representation of the activities in a project. It depicts all

the activities and nodes in the project. On- a network, the arrows terminating at a

node need to be completed before the following activity can begin. Figure 7

shows a sample network consisting of five activities in a project.

Figure 7: Sample Network Schedule

d) Critical Path

The critical path is the longest path through a network. It consists of those

activities that cannot be completed concurrent1y. In other words, the critical path

represents the maximum duration for a project. You can determine the maximum

duration by adding the duration of each activity on the critical path. Typically, a

double line in a network schedule represents the critical path for that project.

Figure 8.6 shows the critical path for a network.

298

Software Project Management (CS615)

Figure 8: Network Schedule with Critical Path Identified

All the activities on the critical path are critical for a project. If an activity on the

critical path is delayed, the entire project is delayed by the same amount of time.

You need to monitor the activities on the critical path because the project depends

on the successful completion of these activities. If required, additional resources

can be applied to these activities to shorten the project duration. However, some

of these activities may also depict a float period.

8.6

Rules for Creating a Network Schedule

There are a few basic rules that are followed while constructing a network

schedule.

1. Each activity has a preceding node and a succeeding node.

2. Each node has a distinct number. As a convention, the number that is assigned

to the head of the arrow is greater than the number that is assigned to the tail.

3. The network schedule has no loops. For example, in Figure 9, activity 1 is the

predecessor of both activity 2 and activity 3. This places activities l, 2, and 3

in a loop.

Fig 9: A loop is not permitted

4. Each activity has a unique preceding and succeeding event associated with it.

For example, in Figure 10, activities A and B have common preceding and

succeeding events associated with them. This is not allowed in a network

schedule.

299

Software Project Management (CS615)

Figure 10: Activities with Common Preceding and Succeeding Events

At times, you need to introduce a dummy activity in a network schedule. This is

an imaginary activity that enables the network schedule to display parallel

activities. For example, in Figure 11, consider A and B as two parallel activities

that can be executed simultaneously. Both A and B must end so that activity C

can begin. A dummy activity is introduced as a dashed arrow to mark the start of

activity C. The dummy activity is introduced to show the dependency between

activities in the network schedule and does not have a description or duration.

Figure 11: Dummy Activity

300

Software Project Management (CS615)

8.7

Using PERT to Schedule a Project

PERT was developed in 1957 to cater to the needs of the Polaris Fleet Ballistic

Missile project of the US government. PERT uses a probabilistic approach to time

estimates. You normally apply it to projects that are characterized by uncertainty.

For example, in a complex software project where you require research to identify

activities. PERT allows you to account for the uncertainties that are common to

most software projects. PERT uses the network schedule to represent a project

schedule while taking the uncertainties into account.

Time Estimates in PERT

PERT is a probabilistic technique that uses three time estimates: It assumes

that activity times are represented by a probability distribution. To finish an

activity, it bases the probability distribution of activity time on three time

estimates:

Optimistic time

Pessimistic time

Most likely time

The optimistic time is the shortest time period within which an activity can

end if everything goes well.

The pessimistic time is the time that an activity takes to complete if

everything that can go wrong goes wrong. This is the longest time that an

activity can take to complete.

The most likely time is the estimate of the normal time that an activity takes to

complete.

From these three estimates, you derive the expected time to complete an

activity. The expected time is also referred to as the average time for the

activity.

To calculate the expected time for each activity, you use the following

equation:

T 0 + 4T m + T p

Te =

Where Te is the expected time, T0 is the optimistic time, Tm is the most likely

time, and Tp is the pessimistic time required to complete an activity.

301

Software Project Management (CS615)

According to this equation, you calculate the expected time for an activity as a

weighted mean of the optimistic, most likely, and pessimistic times. The

weights attached to these times are 1, 4, and 1, respectively.

Consider the example of the activities in a software project as given in Table

1. The time estimates provided in Table 1 are the estimated times for each

activity. Table 2 provides the break-up of the estimated times into the

optimistic, pessimistic, and most likely times for each activity.

Table 2: Optimistic, Most Likely Time, and Pessimistic Estimates for Activities

Optimistic Time

Most Likely

Pessimistic

Tasks

Estimates

Time Estimates

(person days)

Requirement analysis and project planning

Setting up the environment

Software construction

100

Unit testing

User training

System testing

User documentation

Data migration

Conducting user acceptance test

Using the formula for calculating estimated time for an activity, the estimated

time for requirements analysis and project planning is:

7 + 4 ( 10 ) + 13

= 60 / 6 = 10 persondays

Te =

Similarly, you can calculate the estimated times for all the activities in Table 2

using the above formula.

Figure 12 shows the PERT network schedule created using the time estimates.

302

Software Project Management (CS615)

Software

System

Unit Testing

Construction

Testing

Requirements

Analysis and

Project

Planning

User

Acceptance

User Training

Testing

User

Documentation

Data

Migration

Node

Activity

Number

Dummy

Activity

Figure 12: PERT Network Schedule for Activities

ii.

PERT packages and enhancements

Some enhanced versions of the PERT chart support additional planning

activities, such as personnel assignment, resource allocation and cost analysis.

The chart can then draw attention to situations where personnel are assigned

more responsibilities than they can handle, or where allocation of resources

conflicts.

An interesting adaptation of PERT, called flow graph representation, which

was developed by Riggs and Jones (1990), uses precedence networks to

perform project life cycle cost analysis. The flow graph technique analyzes

project costs based on relationships between quantities, unit cost, time

variables, staffing costs and learning etc., all of which are represented on the

PERT- like chart.

The flow graph representation technique places a significant amount of

information on the network graph. This information, just like the basic PERT

information, must be kept constantly updated. A small change to a large PERT

chart can require the complete redrawing of the chart and the recalculation of

the critical path. The resulting tedium does not promote much enthusiasm for

303

Software Project Management (CS615)

keeping the chart updated. For this reason, many computerized PERT utilities

have been developed.

PERT software packages have been available for many years, but it is only

during the past few years that good professional PERT packages have become

available on PCs and other small computers. These packages take much of the

tedium out of the preparation of PERT charts, and also come with additional

features such as various planning analyzers for activity assignment, what if

scenarios and resource allocation.

Computer utilities have been developed to perform flow graph representation

analysis which produces scheduled costs for major project activities2. These

utilities have proven invaluable for project managers and release managers

from laborious desk work, providing them with more time to actively manage

the project.

304

Table of Contents: