Use of Network Techniques

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Use of Network Techniques

CPM and PERT


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Introduction

In 1957, DuPont developed a project

management method designed to addressthe challenge of shutting down chemical

plants for maintenance and then restarting

the plants once the maintenance had been

completed. Given the complexity of theprocess, they developed the Critical Path

Method (CPM) for managing such projects.


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Benefits

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 tomaintaining the schedule and which are not.


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CPM Model

CPM models the activities and events of a

project as a network. Activities are depicted as nodes on the

network and events that signify the beginning

or ending of activities are depicted as arcs or

lines between the nodes.

The following is an example of a CPM

network diagram:


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CPM Diagram


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CPM chart with 8 milestones and 9 activities.


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Steps in CPM Project Planning

Specify the individual activities.

Determine the sequence of those activities. Draw a network diagram.

Estimate the completion time for each activity.

Identify the critical path (longest path through the

network) Update the CPM diagram as the project

progresses.


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1.Specify the Individual Activities

From the work breakdown structure, a listing

can be made of all the activities in theproject. This listing can be used as the basis

for adding sequence and duration

information in later steps.


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3. Draw the Network Diagram

Once the activities and their sequencing

have been defined, the CPM diagram can bedrawn.


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4. Estimate Activity Completion Time

The time required to complete each activity

can be estimated using past experience orthe 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 anactivity's time estimate.


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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 pathanalysis is an important aspect of project

planning.


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|Critical path can be identified bydetermining the following fourparameters for each activity:

ES - earliest start time: the earliest time at which the activitycan start given that its precedent activities must be completed

first.

EF - earliest finish time, equal to the earliest start time for theactivity plus the time required to complete the activity.

LF - latest finish time: the latest time at which the activity canbe completed without delaying the project.

LS - latest start time, equal to the latest finish time minus thetime required to complete the activity.


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The slack time

The slack time for an activity is the time

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

Slack is the amount of time that an activity

can be delayed past its earliest start or

earliest finish without delaying the project.


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The critical path is the path through the

project network in which none of the activitieshave slack, that is, the path for which ES=LS

and EF=LF for all activities in the path. A

delay in the critical path delays the project.

Similarly, to accelerate the project it isnecessary to reduce the total time required

for the activities in the critical path.


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6. Update CPM Diagram

As the project progresses, the actual task

completion times will be known and thenetwork 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 requirementschange.


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CPM Limitations

CPM was developed for complex but fairly

routine projects with minimal uncertaintyin the project completion times. For less routine projects there is more uncertainty in

the completion times, and this uncertainty limits theusefulness of the deterministic CPM model.

An alternative to CPM is the PERT project planningmodel, which allows a range of durations to bespecified for each activity.


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PERT

While CPM is easy to understand and use, it

does not consider the time variations thatcan have a great impact on the completion

time of a complex project.

The Program Evaluation andReview

Technique (PERT) is a network model thatallows for randomness in activity completion

times.


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The Network Diagram

In a project, an activity is a task that must be

performed and an event is a milestone marking thecompletion 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 activityon arcnetwork, in which the activities are represented on

the lines and milestones on the nodes


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Example of a PERT diagram


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The milestones generally are numbered so

that the ending node of an activity has ahigher number than the beginning node.

Incrementing the numbers by 10 allows fornew ones to be inserted without modifyingthe numbering of the entire diagram.

The activities in the above diagram arelabeled with letters along with the expectedtime required to complete the activity.


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Steps in the PERT Planning Process

Identify the specific activities and milestones.

Determine the proper sequence of the activities. Construct a network diagram.

Estimate the time required for each activity.

Determine the critical path.

Update the PERT chart as the project progresses.


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1. Identify Activities and Milestones

The activities are the tasks required to

complete the project. The milestones are theevents marking the beginning and end of one

or more activities.

It is helpful to list the tasks in a table that in

later steps can be expanded to includeinformation on sequence and duration.


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2. Determine Activity Sequence

This step may be combined with the activity

identification step since the activity sequenceis evident for some tasks.

Other tasks may require more analysis to

determine the exact order in which they must

be performed.


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3. Construct the Network Diagram

Using the activity sequence information, a networkdiagram 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 aredepicted 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 informationinto a network diagram.


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4. Estimate Activity Times

Weeks are a commonly used unit of time for

activity completion, but any consistent unit oftime can be used.

A distinguishing feature of PERT is its ability

to deal with uncertainty in activity completion

times.


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For each activity, the model usually

includes three time estimates

Optimistic time - generally the shortest time in which the activitycan be completed. It is common practice to specify optimistic

times to be three standard deviations from the mean so thatthere is approximately a 1% chance that the activity will becompleted within the optimistic time.

Most likely time - the completion time having the highestprobability. Note that this time is different from the expectedtime.

Pessimistic time - the longest time that an activity might require.Three standard deviations from the mean is commonly used forthe pessimistic time.


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PERT assumes a beta probability distribution for the timeestimates. For a beta distribution, the expected time for each

activity can be approximated using the following weightedaverage:

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, ifthree standard deviation times were selected for the optimisticand pessimistic times, then there are six standard deviationsbetween them, so the variance is given by:

[ ( Pessimistic - Optimistic ) / 6 ]2


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5. Determine the Critical Path

The critical path is determined by adding the timesfor the activities in each sequence and determining

the longest path in the project.

The critical path determines the total calendar timerequired for the project. If activities outside thecritical path speed up or slow down (within limits),the total project time does not change.

The amount of time that a non-critical path activitycan be delayed without delaying the project isreferred to as slack time.


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If the critical path is not immediately obvious,

it may be helpful to determine the followingfour quantities for each activity:

ES - Earliest Start time

EF - Earliest Finish time

LS - Latest Start time

LF - Latest Finish time


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These times are calculated using the expected time for therelevant activities. The earliest start and finish times of each

activity are determined by working forward through the networkand determining the earliest time at which an activity can startand finish considering its predecessor activities. The latest startand finish times are the latest times that an activity can startand finish without delaying the project. LS and LF are found byworking backward through the network. The difference in the

latest and earliest finish of each activity is that activity's slack.The critical path then is the path through the network in whichnone of the activities have slack.


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The variance in the project completion time can be

calculated by summing 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 assumptionholds if the number of activities in the path is large

enough for the central limit theorem to be applied.


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Since the critical path determines the

completion date of the project, the projectcan 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 isreferred to as project crashing.


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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 maybe modified to reflect the new situation.


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Benefits of PERT

PERT is useful because it provides the followinginformation:

Expected project completion time.

Probability of completion before a specified date.

The critical path activities that directly impact thecompletion time.

The activities that have slack time and that can lendresources to critical path activities.

Activity start and end dates.


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Limitations

The activity time estimates are somewhat subjective anddepend on judgement. In cases where there is little experiencein performing an activity, the numbers may be only a guess. Inother cases, if the person or group performing the activityestimates the time there may be bias in the estimate.

Even if the activity times are well-estimated, PERT assumes abeta distribution for these time estimates, but the actualdistribution may be different.

Even if the beta distribution assumption holds, PERT assumes

that the probability distribution of the project completion time isthe same as the that of the critical path. Because other pathscan become the critical path if their associated activities aredelayed, PERT consistently underestimates the expectedproject completion time.


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The underestimation of the project

completion time due to alternate pathsbecoming 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 optimisticbias in the expected project completion time.

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Use of Network Techniques