Chapter 6Scheduling

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Learning Objectives Estimate the duration for each activity Establish the estimated start time and

required completion time for the overall project

Calculate the earliest times at which each activity can start and finish, based on the project’s estimated start time

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Learning Objectives (Cont.) Calculate the latest times by which each

activity must start and finish in order to complete the project by its required completion time

Determine the amount of positive or negative slack

Identify the critical (longest) path of activities

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Real World Example Vignette: The World Cup Tournament  South Africa is the host country for the Federation Internationale

de Football Association (FIFA) World Cup tournament in 2010. The South African government is projected to invest more than

R400 billion on infrastructure projects. Additionally, between R2 billion and R5 billion will be spent on

information and communication technology projects. Large investments will be made to ensure all transit systems

form an effective public transport network. Additionally $1.1 billion has been earmarked for new and

renovated stadiums to hold the games of the World Cup. Unfortunately there are reports of schedule delays and budget

overruns. “Many people do not fully understand the details of project management as a specific entity – such as detailed work breakdown structures and critical paths.”

Link 1 Link 2

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Real World Example Vignette: Fast-Track Innovation in Indiana The major improvements necessary on a stretch of the

combined I-65 and I-70 arteries posed numerous challenges to the Indiana Department of Transportation (INDOT).

Project would involve 33 bridges and 35 lane miles of highway, and one side of the highway would need to be shut down.

The project, was completed in just 55 days, earning its name, ‘‘Hyperfix.’’

To decrease the duration of construction time, the whole stretch of affected highway would be closed and numerous contractors would be working each day, 24 hours a day, 7 days a week.

Excellent project management at all phases of this project resulted in successfully and quickly improving the highways, while minimizing any inconvenience to the drivers. Massive communications effort Met with community stakeholders for buy-in

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Activity Duration Estimates The first step in scheduling is to estimate

how long each activity will take. The duration estimate is the total elapsed

time for the work to be done PLUS any associated waiting time.

The person responsible for performing the activity should help make the duration estimate.

Activity Duration Estimates (contd) An activity’s duration estimate must be based on

quantity of resources expected to be used. Estimate should be aggressive, yet realistic.

Playing the game of inflating or padding duration estimates is not a good practice.

Over the life of a project that involves many activities, delays and accelerations will tend to cancel one another out.

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Project Start and Finish Times It is necessary to select an estimated start

time and a required completion time for the overall project.

These two times define the overall window (envelope) of time in which the project must be completed.

The project’s required completion time is normally part of the project objective and stated in the contract.

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Schedule Calculations A project schedule includes:

the earliest times (or dates) at which each activity can start and finish, based on the project's estimated start time (or date)

the latest times (or dates) by which each activity must start and finish in order to complete the project by its required completion time (or date)

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Earliest Start and Finish Times Earliest start time (ES) is the earliest time

at which a particular activity can begin. Earliest finish time (EF) is the earliest time

by which a particular activity can be completed.

EF = ES + Duration Estimate

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Earliest Start and Finish TimesRule #1 The earliest start time for an activity must

be the same as or later than the latest of all the earliest finish times of all the activities leading directly into that particular activity.

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The earliest start time for an activity must be the same as or later than the latest of all the earliest finish times of all the activities leading directly into that particular activity

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Latest Start and Finish Times Latest finish time (LF) is the latest time an

activity must be finished in order for the entire project to be completed by its completion time.

Latest start time (LS) is the latest time an activity must be started in order for the entire project to be completed by its completion time.

LS = LF – Duration Estimate

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Latest Start and Finish TimesRule #2 The latest finish time for a particular

activity must be the same as or earlier than the earliest of all the latest start times of all the activities emerging directly from that particular activity.

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The latest finish time for a particular activity must be the same as or earlier than the earliest of all the latest start times of all the activities emerging directly from that particular activity

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Total Slack, Defined Total slack (TS) or float is the difference

between the calculated earliest finish time of the very last activity and the project’s required completion time.

Total Slack = LF - EF or Total Slack = LS - ES

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Total Slack (Cont.) If total slack is positive, it is the maximum

time the activities on the path can be delayed.

If total slack is negative, it is the amount of time the activities on the path must be accelerated.

The total slack for a particular path of activities is common to and shared among all the activities on that path.

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Critical Path The critical path is the longest path in

the diagram. The activities that make up the critical

path have the least slack. All activities with this value are on the

critical path.

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Types of Critical Paths

Noncritical paths have positive values of total slack.

Critical paths have zero or negative values of total slack.

The most critical path is the longest critical path.

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Free Slack The amount of time an activity can be

delayed without delaying the start of other activities.

It is the relative difference between the amounts of total slack for activities entering into the same activity.

It is always a positive value.

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Scheduling for Information System Development Some common problems that push IS

projects past their required completion time:Failure to identify all user requirementsLogical design flawsContinuing growth of project scopeUnderestimating learning curves for

new software packages

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Project Management Software

Allows one to perform scheduling functions.

Activity durations can be estimated in a variety of ways.

Project start and finish times can be entered in a variety of ways.

Can calculate dates, times, total and free slack.

Appendix 1Probability Considerations

(will not cover for undergraduate class)

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Probability Considerations Activity Duration Estimates

Optimistic time: time to complete an activity if everything goes perfectly well.

Most likely time: time to complete an activity under normal conditions.

Pessimistic time: time to complete an activity under adverse circumstances.

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Probability Considerations The Beta Probability Distribution

When using three time estimates, it is assumed that they follow a beta probability distribution.

The expected duration is calculated using the following formula:

te = (to + 4(tm) + tp) / 6

Peak represents most likely time; divides curve into two equal parts

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Probability Considerations Probability Fundamentals

Network planning that uses three time estimates for each activity can be considered a stochastic or probabilistic technique, since it allows for uncertainty.

Any technique that uses only one time estimate is considered to be a deterministic technique.

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Probability Fundamentals (Cont.) The total probability distribution is a

normal probability distribution. The variance for the beta probability

distribution of an activity is: Variance = 2 = ((tp – to) / 6)2

The standard deviation, , is another measure of the dispersion of a distribution and is equal to the square root of the variance.

Standard deviation measures dispersion

Normal Distribution

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Probability Fundamentals (Cont.) The total probability distribution of the

critical path activities is a normal distribution.

The mean equals the sum of the individual activity expected durations.

The variance equals the sum of the individual activity variances.

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Calculating Probability The probability of completing a project

before its required completion time: Z = (LF – EF / t)

LF = the required completion time (latest finish). EF = the earliest expected finish time (mean of

the normal distribution). t = the standard deviation of the total distribution

of activities on the longest path. Z = number of standard deviations between EF

and LF on the normal probability curve

99%95%

68%50%

50% 42.922%