pm pert & crashing 7
TRANSCRIPT
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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 .