Copyright © 2009 T.L. Martin & Associates Inc. Chapter 2 Key CPM Scheduling Terms

Copyright © 2009Copyright © 2009

T.L. Martin & Associates Inc.T.L. Martin & Associates Inc.

Chapter 2Chapter 2 Key CPM Scheduling Terms Key CPM Scheduling Terms



The CPM ScheduleThe CPM Schedule

• A critical path method schedule network is a model of how a project can be performed.

• Critical Path – The chain of activities in a network that requires the longest amount of time for performance.

• There are two graphical techniques for representing a network.

• ADM and PDM.




• ADM - Arrow Diagramming Method.

• ADM - activity on arrow.

• The tail of the arrow depicts the start of activity.

• The description is along the arrow’s shaft.

• The arrowhead denotes work completion.



Arrow Diagramming MethodArrow Diagramming Method

10 25

2015

30Form Walls

5 Days

Pour Wall Concrete

1 Day

Place WallRebar

3 Days

Logi

c Ti

e0

Days

Logic Tie

0 Days

An Arrow Diagram.




• PDM - Precedence Diagramming Method.

• PDM - activity on node.

• The description is placed in a box

• Connecting lines have arrowheads showing direction.



Precedence Diagramming Precedence Diagramming MethodMethod

Form Walls

5 Days

Place Wall Rebar

3 Days

Pour Wall Concrete

1 Day

A Precedence Diagram.



The CPM ScheduleThe CPM Schedule• Creation of the ADM or PDM graphical diagram occurs

during the initial planning of the project. • The project is broken up into meaningful work elements

or Activities. • The duration or performance period for each activity is

developed based on a reasonable production rate for an accurate quantity of work.

• Predecessor – An activity that comes before another activity that is logically linked to it.

• Successor - An activity that comes after another activity that is logically linked to it.




• Physical, contractual and optional activity relationships or logic are included.

• The activity data and relationships are used as the basis for the computation dates based on forward and backward passes through the activity network.

• Forward Pass - The computation procedure that determines the early occurrence time of an activity.

• Backward Pass – The computation procedure that determines the late occurrence time of an activity.

• Float (Total) – Is the amount of time an activity can be delayed before its performance will affect project completion.

• Free Float – Is the amount of time an activity can be delayed before its performance will affect the float of another activity.



Forward PassForward Pass

* Dates are determined by calendar planning units

Driving Relationship

4

1* 4

4

5 8

2

5 6

4

9 12

A

B

C

D




• Early Dates - The earliest time an activity can take place.• Late Dates - The latest time an activity can take place.• Calendar – A specification that defines work and non-work days

during computation of activity performance dates.• Driving relationship - actual (or imagined) dependency between

work activities that causes the calculated early start of a successor activity.

• Note: If late dates are missed - Project Completion Slips– Recovery Schedule– Acceleration Order– Grant a Time Extension



Backward PassBackward Pass

Start pass at latest calculated

early finish

4

9 12

4

5 8

2

7 8

4

1 4

A

B

C

D



Total FloatTotal Float

• The number of planning units an activity can be delayed without affecting project completion

• Total Float can be calculated by start dates or finish dates

• Total Float is the late start minus the early start



Total FloatTotal Float Calculation CalculationStartStart

LS - ES = TFA 1 1 0B 5 5 0C 7 5 2D 9 9 0



Total FloatTotal Float

• The number of planning units an activity can be delayed without affecting project completion

• Total Float can be calculated by start dates or finish dates

• Total Float is the late start minus the early startOR

• Total Float is the late finish minus the early finish



Total FloatTotal Float Calculation CalculationFinishFinish

LF - EF = TFA 4 4 0B 8 8 0C 8 6 2D 12 12 0



Free FloatFree Float

• Free Float is not to be confused with Total Float

• Free Float is the amount of time an early start or finish can be delayed without delaying the start of a successor activity

• Free Float can never be less than zero



Free FloatFree Float

Can slip 2 days before impacting successor activity

5

5

5

5

5

102

1 5

6 10

15

20

25

11

16

21

6 152 3



Critical PathCritical Path

• The longest path from project start through project finish with the least amount of float

• Critical Path for an original or baseline schedule should have TF=0

• Can be positive or negative in update• Critical Path through activities with multiple

calendars might not have same Total Float value• Total Float calculations can be manipulated by

constraints and computation methods.




• If there are activity errors, the computer generated start and finish dates for the project tasks will also be in error.

• This is the rule of GIGO – Garbage In causes Garbage Out.

• The biggest problem with using a computer in scheduling is that it camouflages garbage.




• The GIGO rule requires an analysis of the fundamental activity building blocks – work quantities – production rates – task relationships.

• If there are errors the resulting activity schedule will be unreliable.




• CPM allows control of work progress. • CPM allows for prompt schedule recalculations.

– Test different logic scenarios – Test various applications of men– Test different allowances of equipment and/or

materials • The impact of a contract modification can be measured,

prior to the actual direction to precede.




• Update – The process of revising network activity data to reflect actual progress, revisions to performance plan, changes to the work or all of the proceeding.

• A properly updated CPM schedule allows:– Measurement of contractor performance. – Delay claim proof or disproof .

• An update can result in:– Withholding monies from the contractor– Contractor Default Termination – A Change Order for additional performance time




• Status Date – The date up to which progress is included into a schedule update.

• Out-of-Sequence – This occurs when a successor is progressed prior to a predecessor activity, contrary to network logic.

• Loop – A set of activities arranged by improper logic so that the chain has no start or end.




• The updating process includes:– Current project status

• completed quantities of work• resources used as of the status date• agreed to percent of payment• activity actual start and finish dates• any necessary or beneficial logic revisions

– Changes– Time Extensions

• The GIGO rule applies to the computer generated CPM schedule updates.




• Attempts may be made to manipulate the schedule computation.

• Computer CPM software allows many options for data manipulation.

• Constraint – A forced manual override of computed activity timing and/or float.

• Computation methods and activity constraints should be agreed to with the original schedule submission.

• Constraints and computation method should not be revised without prior agreement.



Computer SoftwareComputer Software

• Bar Charts– Spread Sheet Programs - Lotus 1-2-3 & Excel– Scheduler 7

• CPM - PDM– Primavera Project Planner – Sure-Track by Primavera– Microsoft Project



Please Proceed to Please Proceed to

Chapter 3 - Requirements of Chapter 3 - Requirements of a realistic CPM schedule.a realistic CPM schedule.

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Copyright © 2009 T.L. Martin & Associates Inc. Chapter 2 Key CPM Scheduling Terms