Chapter 6 Scheduling

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

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 3

Real World Example

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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 4

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 5

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 .

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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) 10

Earliest Start and Finish Times

   Earliest start time (ES) is the earliest time at which a particular activity can begin.

Earliest finish time time by which a particular activity can be completed.

(EF) is the earliest EF = ES + Duration Estimate 11

Earliest Start and Finish Times Rule #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 13

Latest Start and Finish Times

   Latest finish time completion time.

(LF) is the latest time an activity must be finished in order for the entire project to be completed by its 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 14

Latest Start and Finish Times Rule #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 16

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 17

Total Slack (Cont.)

   If total slack is can be delayed.

positive , it is the maximum time the activities on the path If total slack is accelerated.

negative , it is the amount of time the activities on the path must be

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 the diagram.

is the longest path in 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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0 2 1 -7 A 2 -5 2 2 2 B 5 3 5 2 -5 3 C 6 4 -1 5 5 5 E 7 2 7 9 6 4 -1 D 9 3 2 6 2 F 14 5 7 14 8 7 9 H 5 12 13 7 8 G 19 4 12 19 22 9 12 I 3 15

Scheduling for Information System Development

 Some common problems that push IS projects past their required completion time:  Failure to identify all user requirements    Logical design flaws Continuing growth of project scope Underestimating learning curves for new software packages 24

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.

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Appendix 1 Probability Considerations

(will not cover for undergraduate class)

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Probability Considerations Activity Duration Estimates

  

Optimistic time:

activity if everything goes perfectly well.

time to complete an

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:

t

e = (

t

o + 4(

t

m ) +

t

p ) / 6

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

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 = s 2 = ((

t

p The standard deviation , – s

t

o ) / 6) 2 , is another measure of the dispersion of a distribution and is equal to the square root of the variance. 31

Normal Distribution Standard deviation measures dispersion

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 / s t ) LF = the required completion time (latest finish).

  EF = the earliest expected finish time (mean of the normal distribution).

s 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 34

50% 68% 95% 99% 50% 42.922%