pm pert & crashing 7

7/29/2019 PM PERT & Crashing 7

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Upendra Kachru PROJECT MANAGEMENT

PERT & Project Crashing


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Revision - Scheduling?

Identify all activities

Determine logical order in a network diagram

Assign resources to each activity

Estimate time required for that activity

Compare emerging schedule with imposed dates

Consider project budget and cash flow, quality demands, and risk

factors


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Revision - Scheduling?

Inaccuracy in

work

instructions

Consider time constraints

Unexpected

meetings

Interruptions

Emergencies/ill

ness

Vacation

Rework

Resources or

information not

available on time


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Establish Change Control

Projects are conducted in an atmosphere of uncertainty

Plans must be made for dealing with change

baseline the approved plan for a project plus or minus approvedchanges. PMBOK Guide

Change control system a collection of formally documented

procedures that define how project deliverables anddocumentation will be controlled, changed, and approved.

PMBOK Guide


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Establish Change Control

Document potential changes to a project with a change request Every change to a project must be formally proposed

Change request request to expand or reduce the projectscope, modify policies, processes, plans, or procedure, modify

costs or budgets, or revise schedules. PMBOK Guide


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Change Request Form


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Critical Path

From the CPM diagram of the Garden project given to you

Identify the critical path Determines the earliest possible end date of the project

Most critical in terms of time

Methods for determining the critical path


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Uncertainty in Project Schedules

Construct the best possible schedule

Manage the project very closely

OR

Estimate a range of possible times each individual activity may take

Examine the impact of each activity on the entire schedule

Use PERT for this approach


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Program Evaluation and Review Technique

(PERT)

Developed for use in the aerospace industry - mostfrequently used in R&D programs

Aid to understanding how variability in the duration

of individual activities impacts the entire project

schedule Sequence activities into a network


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Program Evaluation and Review

Technique (PERT)

Calculate expected time (TE) for each activity

Distribution of each activity time based on a BETA probabilitydistribution

Create Accounts for uncertainties:

Optimistic TO Most likely TM Pessimistic TP

Expected project length is approximated by a normal distribution

(mean, standard deviation)


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Program Evaluation and Review

Technique (PERT)

PERTWeighted =

Average 6

Optimistic + (4 xMost Likely) + Pessimistic

PERTStandard =

Deviation 6

Pessimistic - Optimistic


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Using PERT

Activity LowMost

LikelyHigh Mean Sigma Var.

a m b x 2

Initial Draft

Gather Information 40 45 80 50.0 6.7 44.4

Write Sections 35 50 100 55.8 10.8 117.4

Review Informally 10 15 30 16.7 3.3 11.1

Inspections

Inspectors Inspect 18 25 50 28.0 5.3 28.4

Prepare Defects/Issues List 10 20 40 21.7 5.0 25.0

Resolve Defects/Issues 10 25 60 28.3 8.3 69.4

Make Necessary Changes 15 20 40 22.5 4.2 17.4

Estimated Project Totals: 200 223.0 17.7 313.2

Mean = (a + 4m + b) / 6 Variance = [ (ba) / 6 ]2


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PERT Time Estimate Garden Project

ExampleActivity Description Optimistic

Time (a)Pessimistic

Time (b)Most Likely

Time (m)ExpectedTime (te)

Variance(2)

A Pick up Trash 12 18 15 15 1.00

B Fill Fuel 5 5 5 5 0

CFetch Hedge

Clipper3 10 5 5.5 0.25

D Trim Weeds 30 30 30 30 0

E Mow Front 40 55 45 45.83 6.25

F Edge Side walk 14 16 15 15 0.026

G Trim Hedge 30 35 30 30.83 0.69

HMow Back

Yard25 40 30 30.83 6.25

I Bag Grass 20 30 30 28.33 2.76

J Collect Trash 15 15 15 15 0

K To Disposal 40 50 45 45 2.76


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2-Point PERT

Expected value = 5m + b

6

Omits the use of the most optimistic time.

When would this be appropriate?


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PERT Considerations

Advantages

Reinforces the degree of uncertainty that exists in project

schedules

Calculations indicate that expected time is actually longer than

most likely time Difficulties

Takes more effort to create 3 estimates

No guarantee how good the estimates are

May underestimate the risk of a schedule running long


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Initiatives to Improve On-Time

Schedule Delivery


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Actions to Reduce the Critical Path

Shorten the duration of criticalactivities.

Shorten activities by assigning

more resources.

Shorten activities that cost theleast to speed up.

Reduce the project scope and/orquality.

Increase the number of work

hours/days

Overlap sequential activities

Partially overlap sequential

activities

Schedule activities at the same

time.


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Compress the Project Schedule Crashing: Crashing may cost more money to speed up the

schedule

Fast tracking: up the schedule Fast tracking may increase the risk

to speed

Crashing a specific type of project schedule compressiontechnique performed by taking action to decrease the totalproject durationhow to get the maximum schedule durationfor the least additional cost. PMBOK Guide

Fast Tracking a specific project schedule compressiontechnique that changes network logic to perform scheduleactivities in parallel. PMBOK Guide


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Project Crashing

One of the most important concepts in ProjectManagement is crashing. This is a cost-time

tradeoffs.

There is a trade-off curve between the projectcompletion time and the additional cost. There are

always time-cost trade-offs.


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Project Crashing

The project duration is too long, The customer wants to know the additional costs

for saving part of the project completion time,

and

The company may like to minimize the sum ofdirect and indirect project costs without

disturbing the stipulated duration time.


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Crashing Consideration Questions

How fast can the project be completed? To crash the project one day, what activity would be crashed and

what would it cost?

To crash the project two days, what activities would be crashed

and what would it cost in total? If there is a bonus of Rs.125 per day for finishing early, what

would I crash and how fast would I get done?

If there is a bonus of Rs. 225 per day for finishing early, what

would I crash and how fast would I get done?


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Crashing Considerations

A systematic procedure to take such decisions has beendeveloped by Burgess.

Starting with the bottom activity the method makes comparisons, between

the sums of squares of daily resource requirements and selects the one with

minimum sum.

The target always being toward reducing the project duration time with

minimum increase indirect costs.

The process is continued till a step is reached when increase in direct cost

is less then the decrease in indirect costs. That means no further decrease

in total costs is possible.


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Special Terms used in Crashing

Normal Activity Time-Cost-Point: is the lowest point on a time-cost graph and

represents the absolute minimum cost for accomplishing the activity in normaltime. Normal time is the shortest time to perform the activity within the constraint

of minimum direct cost.

Feasible Activity Time-Cost Trade-Off Points: represent the various combinations

of minimum direct costs and their corresponding least timings for one individual

activity only. If you want to schedule within the available float, it is called time-critical

resource leveling, because time is of the essence for your project.

If you minimize resources and continue sliding tasks over until resources become

available, even if it means slipping the end date, it is called resource-critical

leveling.


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Crashing

Certain activities are performed at a faster than normal pace

Which activities

are on the critical

path?

Which critical pathactivity costs the

least on a per day

basis to speed up?


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Time / Cost Tradeoff

The activity time / cost trade-off curve can beconstructed for any project to learn what the costs

of crashing are. There are a number of reasons why

crashingis desired:

i / C d ff C


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Time / Cost Tradeoff Curve

Activity Time/Cost Trade-Off Curve


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Crashing Example Set Up

For example, in the Garden project the critical path is A-E-H-I-K.The corresponding activities are Pick up Trash, Mow Front,

Mow Back Yard, Bag Grass and Disposal.

You would damage the mower if you start mowing without

removing the trash. So, activity A would be difficult to crash.

However, you can mow the front lawn and the back lawn

simultaneously, if

You invest in an additional mower, and

Add an additional man to run the second lawn mower.


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Crashing Example

To asses the benefit of crashing certain activities the following times

and costs need to be known: The normal time (Tn) is the time to compete the activity under normal

conditions.

The normal cost (Cn) is the activity cost associated with the normal time.

The crash time (Tc) is the shortest possible time to complete the activity.

The crash cost (Cc) is the activity cost associated with the crash time.

Cost to crash = CcCn/ (Tn - Tc)

Suppose the cost of renting the lawn mower is ` 250 each day and the cost of an

extra gardener is ` 150. By crashing activity E, we can reduce the total time to 135

minutes. However, we would be able to do this at a crash cost of ` 400.

C hi E l i All C h M d


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Crashing Example in All-Crash Mode

Activity Description ES EF LS LF Slack

A Pick upTrash

0 15 0 15 0

B Fill Fuel 0 5 10 15 10

CFetch Hedge

Clipper0 5 10 15 10

D Trim Weeds 15 45 30 60 15

E Mow Front 15 60 15 60 0

F Edge Sidewalk

15 30 45 60 30

G Trim Hedge 5 35 30 60 25

HMow Back

Yard15 45 15 60 15

I Bag Grass 60 90 60 90 0

JCollect

Trash

60 75 75 90 15

K To Disposal 90 135 90 135 0

C hi i


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CrashingUsing LP

As the Garden project example shows, you start with the activity

which is on the critical path and has the least cost-time slope also,which means that the time can be bought down from this activity at

the cheapest price.

The possible augmentation is affected and new times and slacks are

calculated. Augmentation from activity-to-activity is continued till it

is found that after updating, the network critical path goes sub-

critical. Then find another activity (on some near-critical path)

having the least cost-time slope and then to make the desirable

augmentation. Special care must be taken when the project network

has two or more critical paths.

C hi U i LP


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CrashingUsing LP The incremental augmentation procedure to adjust activity time for

producing the time/cost tradeoff curve does not necessarily provide an

optimal solution. Linear programming is most effectively used in order

to guarantee the lowest additional cost for completing the project by a

specified target date.

nP = Normal time for activity P

mP = Crash time for activity P sP = Cost per unit time of reduction for activity P

yP = Number of units of time by which activity P is shortened

xP = Finish time for activity P

C hi U i LP


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CrashingUsing LP

Then, we wish to minimise the cost sPyP by crashing subject to

the precedence constraints, the target date completion timeconstraint, and constraints on amount of crashing permitted.

Minimise P=xP=AsPyP

Subject to

xP P=x

P=A (xP+ nP- yP) (precedence constraints)

xP t (target date constraint)

yP P=x

P=A (nP- mP) (constraints on amount of crashing permitted)

This approach is used for medium-sized projects, as it may be

computationally expensive for a large network.


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Fast tracking Activities normally performed in series are performed at the same

time


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C l i c k t o e d i t c o m p a n y s l o g a n .

pm pert & crashing 7

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