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Regarding PERT, CPM and PDM

12 replies [Last post]
Mon, 2006-01-09 02:20
Prasath Raga
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Hi all,

I am very sorry that i am going to ask a very very basic question in scheduling.

I know some what about PERT, CPM and PDM. Now i want clear explanation and PERT, CPM and PDM. what is the difference between them, how to calculate early and late start by each method.

I searched on the internet, but i am not able to find clear one. if any one suggest me a good website where i can find clear defination.

And in particular, what method is used by Primevara for calculating early start and early finish.

Thanks in advance
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Replies

Sat, 2017-01-28 12:46
#1
arash molaei
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ِDear Prasath Raga,

As far as i know:

We have two different Diagramming methods: PDM ( Precedence Diagramming Method ) and ADM (Arrow Diagramming Method). As their names suggest, they are diagramming methods.

But:

CPM is a A network analysis technique used to predict project duration by analyzing the sequence and duration of activities.

and:

A PERT chart is a project management tool used to schedule, organize, and coordinate tasks within a project. PERT stands for Program Evaluation Review Technique, a methodology developed by the U.S. Navy in the 1950s to manage the Polaris submarine missile program.

 

Similarities:

CPM and PERT are somehow similar: they both are used to assist in project scheduling.

Difference:

CPM assumes that activity durations are known with certainty, however PERT assumes that activity durations are random variables (i.e.,probabilistic).

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Wed, 2014-07-02 04:54
#2
Ahmad zia Nangialay
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Thanks David,

this is the best ansower you provided in very less words, i had the same confusion which is completely solved after your statement,

 

Regards

Ahmad Zia Nangialay

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Mon, 2006-01-16 06:59
#3
David Bordoli
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This is my simple take on it…

CPM is a generic term for any network technique.

PDM is a specific CPM, particularly ‘activity-on-node’.

PERT is a specific CPM that used three estimations of activity duration but only works with ‘activity-on-line’ type networks (because of the relatively simplistic statistical techniques it uses in calculating risk).

Regards

David
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Sun, 2006-01-15 12:06
#4
Norzul Ibrahim
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Dear Prasath,

Below is the definition that I copied from PMBOK:-

Critical Path Method (CPM) : A network analysis technique used to predict project duration by analyzing which sequence of activities (which path) has the least amount of scheduling flexibility (the least amount of float).Early dates are calculated by means of a forward pass ,using a specified start date .Late dates are calculated by means of a backward pass ,starting from a specified completion date (usually the forward pass calculated project early finish date).

Program Evaluation and Review Technique (PERT) : An event-oriented network analysis technique used to estimate program duration when there is uncertainty in the individual activity duration estimates .PERT applies the critical path method using durations that are computed by a weighted average of optimistic,pessimistic,and most
likely duration estimates.PERT computes the standard deviation of the completion date from those of the path’s activity durations.Also known as the Method of Moments Analysis.

Precedence Diagramming Method (PDM) : A network diagramming technique in which activities are represented by boxes (or nodes ).Activities are linked by precedence
relationships to show the sequence in which the activities are to be performed.

Hope the above will not make yu more confused...

norzul
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Sun, 2006-01-15 11:15
#5
Norzul Ibrahim
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Attached link is the print output example from MS Project 2003 with regards to the PERT Analysis.

PlanningPlanet Blog

norzul
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Sun, 2006-01-15 10:03
#6
Norzul Ibrahim
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Gary,

This is the definition I’ve got from MS Project 2003. In fact there’s a PERT analysis tool in MS Project, where yu need to key in the optimistic, most likely, & pessimistic data. Yes from MSP point of view, looks like that PERT is part of risk analysis.

norzul

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Sun, 2006-01-15 09:53
#7
Gary France
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Norzul,

Yes, PERT does mean Program Evaluation and Review Technique. However, I am not sure that the second part of your definition is correct.

Whilst it has been used to mean different things over the years, in its simplest of terms, PERT means the same as what became known as Critical Path Analysis. Originally it was used to describe the activity on arrow method, but more latterly it is also been used to describe the activity on node method.

The three scenarios you describe do not relate specifically to the PERT process – they are more akin to part of a risk analysis methodology for calculating an overall project duration and the % chance of meeting the end date. I checked the link that Gary (Whitehead) gave us and indeed that describes the three scenarios that you describe, but most CPA / PERT processes do not use this method for determining an activity duration – some individuals might use this method, but it doesn’t necessarily follow.

Gary
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Sun, 2006-01-15 09:03
#8
Norzul Ibrahim
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PERT stands for "Program, Evaluation, and Review Technique"...am I correct?

A PERT analysis is a process by which you evaluate a probable outcome of schedule/task duration based on three scenarios: best-case (optimistic), expected-case, and worst-case (pessimistic)
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Sun, 2006-01-15 05:04
#9
Gary France
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Sonia,

Strictly speaking you are not quite correct when you say "PDM is nothing but Bar chart or gnatt chart model of network...".

To me, PDM means Precedence Diagram Method which is a Network Diagram in the Activity-on-Node format. This is not a Gantt chart as you suggest.

Gary.
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Sun, 2006-01-15 03:19
#10
Sonia Thomas
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Hi Prasanth,

Hope you would have got he details through teh suggested books/links.

However, a quick answer..

CPM & PERT are the scheduling techniques. CPM is an activity oriented method and PERT is event oriented method. PDM (Precedence Diagraming Method) is a network model and not a scheduling technique. PDM is nothing but Bar chart or gnatt chart model of network, where you can specify the relation between a predecessor and successor more in detail using Start-to-Start, start-to-finish, Finish-to-start and Finish-to-finish relationships.

Primavera uses CPM technique for schedule calculation. It gives network in PDM format and network format (PERT chart - not PERT technique).

regards
Sonia
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Tue, 2006-01-10 08:29
#11
Gary Whitehead
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Reasonable explanation of CPM and PERT can be found here
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Mon, 2006-01-09 14:49
#12
Bernard Ertl
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Chris Hendrickson’s book has a good description of ADM, PDM and PERT calculations: Fundamental Scheduling Procedures

Bernard Ertl
InterPlan Systems
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