Transcript C. Key CPM Scheduling Terms

Chapter 2
Key CPM Scheduling Terms
Copyright © 2009
T.L. Martin & Associates Inc.
The 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
Arrow Diagramming Method
Form Walls
10
25
5 Days
1 Day
Lo
gic
Tie
0D
ays
Tie
gic
Lo
ays
0D
15
Pour Wall Concrete
Place Wall
Rebar
3 Days
20
An Arrow Diagram.
Copyright © 2009
T.L. Martin & Associates Inc.
30
The CPM Schedule
•
•
•
•
PDM - Precedence Diagramming Method.
PDM - activity on node.
The description is placed in a box
Connecting lines have arrowheads
showing direction.
Copyright © 2009
T.L. Martin & Associates Inc.
Precedence Diagramming
Method
5 Days
1 Day
Form Walls
Pour Wall Concre te
3 Days
Place Wall Re bar
A Precedence Diagram.
Copyright © 2009
T.L. Martin & Associates Inc.
The 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
Forward Pass
1*
A
4
4
5
8
4
B
5
C
Driving Relationship
6
2
9
D
* Dates are determined by calendar planning units
Copyright © 2009
T.L. Martin & Associates Inc.
12
4
The CPM Schedule
• 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
Copyright © 2009
T.L. Martin & Associates Inc.
Backward Pass
9
4
D
5
8
4
B
7
C
1
A
12
8
2
4
4
Copyright © 2009
T.L. Martin & Associates Inc.
Start pass at
latest calculated
early finish
Total 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
Copyright © 2009
T.L. Martin & Associates Inc.
Total Float Calculation
Start
A
B
C
D
LS
1
5
7
9
-
ES
1
5
5
9
Copyright © 2009
T.L. Martin & Associates Inc.
=
TF
0
0
2
0
Total 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
OR
• Total Float is the late finish minus the early
finish
Copyright © 2009
T.L. Martin & Associates Inc.
Total Float Calculation
Finish
A
B
C
D
LF
4
8
8
12
-
EF
4
8
6
12
Copyright © 2009
T.L. Martin & Associates Inc.
=
TF
0
0
2
0
Free 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
Copyright © 2009
T.L. Martin & Associates Inc.
Free Float
1
5
5
6
10
5
11
15
5
16
20
5
Can slip 2 days before impacting
successor activity
2
3
2
6
21
15
10
Copyright © 2009
T.L. Martin & Associates Inc.
25
5
Critical 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
The CPM Schedule
• 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.
Copyright © 2009
T.L. Martin & Associates Inc.
Computer Software
• Bar Charts
– Spread Sheet Programs - Lotus 1-2-3 & Excel
– Scheduler 7
• CPM - PDM
– Primavera Project Planner
– Sure-Track by Primavera
– Microsoft Project
Copyright © 2009
T.L. Martin & Associates Inc.
Please Proceed to
Chapter 3 - Requirements of
a realistic CPM schedule.
Copyright © 2009
T.L. Martin & Associates Inc.