AMERICAN SPORTS
BUILDERS ASSOCIATION
CRITICAL PATH METHOD
SCHEDULING FOR SUCCESS
December 6-8, 2008
New Orleans, Louisiana
Presented By: Bill Pronevitch
SCHEDULING- Is It Necessary?
• Effective Project Management involves
coordinating activities such as:
–
–
–
–
Planning
Organizing
Controlling Time (scheduling)
Cost
• The “Scheduling” process forces people to:
– Quantify their effort in discrete terms
– Place tasks in proper relationships
SCHEDULING- Methodologies
• Two of the most common methodologies:
– Bar Charts
– Critical Path
• ADM: Arrow Diagramming Method
• PDM: Precedence Diagramming Method
• Both scheduling methods are widely used
for:
– Controlling
– Making optimal use of project time
SCHEDULING- Bar Charts
• Bar Charts (also called “Gantt Charts”) are
primarily for controlling Time elements:
– Analyze/ specify the basic approach to be used
– Segment the work into a reasonable number of
activities that can be scheduled
– Estimate the time required to perform each activity
(i.e.- activity duration)
– Place activities in “time-order” (logic)
– Adjust the chart until the specified completion date, if
one exists, is satisfied.
SCHEDULING- Bar Chart
A
Gantt Chart
(Bar Chart) shows:
B
•All Activities and
Status on a Single
Chart
C
D
E
F
0
1
2
3
4
5
6
7
8
9
10
11
12
SCHEDULING- Bar Charts
• Benefits:
– Plan, Schedule, and Progress on a single chart
– A simple, understandable way to schedule small
projects or undertakings
• Disadvantages:
– Activity-limited, can’t handle complex projects
– Doesn’t show logic ties (activity relationships)
– Insufficient detail to enable timely detection of
schedule slippages on long duration activities
SCHEDULING: Critical Path
• The Critical Path Method (CPM) was designed
for and is useful on projects where the duration
of each activity can be
with
reasonable certainty
- it predicts project overall completion
- it identifies the (critical) activities that
control the overall length of the project
• CPM is widely used in: process industries
construction and plant outages/ shutdowns.
SCHEDULING: Critical Path
• Benefits:
– Determines shortest project completion time
– Identifies “critical” activities – that can not be
slipped or delayed (i.e.- “TOTAL FLOAT”)
– Shows allowable slippage for “non-critical”
activities (i.e.- “FREE FLOAT”)
• Disadvantages:
– Large number of activities required
– Difficult to read, understand, and maintain.
SCHEDULING: CPM/ADM
• ADM (Arrow Diagramming Method):
– Arrow (line): represents an Activity
– Tail (of arrow): shows an Activity’s start
– Head (of arrow): shows an Activity’s finish
– Node (or event): shown at each end
• The “Activity Number” consists of Head
and Tail numbers, commonly referred to as
I-J nodes (i.e.- “I-J Method” of CPM)
SCHEDULING: CPM/ADM
ADM: Arrow Diagramming Method
Process Work
Requisition
Install
Order
Material
Pump
1
4
2
Assign
Crew
3
5
(Dummy Activity – shows
relationship, zero duration)
SCHEDULING: CPM/ADM
• Benefits:
– Allows use of “Dummy Activities” which:
• Have a “ZERO” time duration
• Can be used to show additional relationships
• Disadvantages:
– An Activity’s “Predecessor” must be complete
before the Activity can start!
– Neither the Activity’s “arrow length” or “arrow
direction” have any meaning
SCHEDULING: CPM/PDM
• PDM (Precedence Diagramming Method):
– Box/ Node: represents an Activity
– Line/ Arrow: represents a Precedence – with time and
direction properties
• Precedence consist of two parts:
– “Relationship”: a Predecessor or Successor
– “Lag”: the (positive or negative) number of work
periods by which a specified Activity will be delayed
(assumed to be “0” if not specified)
SCHEDULING: CPM/PDM
PDM: Precedence Diagramming Method
Process
Work
Order
Requisition
Material
Install
Pump
A
B
C
Assign
Crew
D
Pump
Installation
Completed
E
SCHEDULING: CPM/PDM
• Types of Relationships, Lags (aa,n):
– FS, 0: Finish-to-Start, With “0 units” of delay
– FF, 7: Finish-to-Finish, with “7 units” of delay
– SS, 5: Start-to-Start, with “5 units” of delay
– SF, 0: Start-to-Finish, with “0 units” of delay
SCHEDULING – PDM/ LOGIC
Relationships (Logic Ties, Lags
Finish -to- Finish
Finish –to- Start
A
(FF,5)
A
B
B
(FS,0)
Start –to- Start
A
Start –to- Finish
B
B
A
(SS,4)
(SF,0)
SCHEDULING: CPM/PDM
• “6” Major Types of Constraints
– Start On: a mandatory date
– Start No Earlier Than*: can start “later”
– Start No Later Than*: can start “earlier”
– Finish On: a mandatory date
– Finish No Earlier Than*: can finish “later”
– Finish No Later Than*: can finish “earlier”
* NOTE: Schedule logic drives “earlier” and
“later” start and finish dates
SCHEDULING- PDM/ OTHER
• Hammock Activity
– Spans many activities to show an “overall”
performance period (i.e.- a summary activity)
• Logic Loop
– One of the most common “scheduling errors”
– Occurs when a set of activities precede each
other in a “circular” fashion, (a group of
activities can never begin nor end)
– Must be eliminated prior to calculating the
schedule (i.e.- running a time analysis)
SCHEDULING- Hammock
PDM: Hammock (Summary) Activity
A
B
C
D
E
SCHEDULING – PMD/LOOP
LOOP: No beginning or end
(a circular path)
A
B
C
LOOP
F
E
D
SCHEDULING – PDM/ CALC’S
• Once a network has been created and the
durations estimated for each activity, both
the “Total Time” to reach project
completion and each activity’s “individual”
start and finish times, can be calculated
– Manual computation is easy and logical (but
tedious & time-consuming on large projects)
• Forward Pass: calculates early start/finish, (ES/EF)
• Backward Pass: calculates late start/finish, (LS/LF)
SCHEDULING – CALC’S
• FORWARD PASS:
– Calculates “Earliest Start” and “Earliest
Finish” times, observing the following rules:
• Assign an “ES” time to the first activity
• Other activities start as soon as their
“predecessor” relationships have been satisfied
• Equations:
– ES = EF (Pred.) + 1
– EF = ES + Duration - 1
SCHEDULING – CALC’S
• BACKWARD PASS:
– Calculates “Latest Start” and “Latest Finish”
times, observing the following rules:
• Assign an “EF” time to the last activity.
• All activities finish as soon as their “Successor”
relationships have been satisfied.
• Equations:
– LF = LS (Succ.) – 1
– LS = LF – Duration + 1
SCHEDULING – CALC’S
• PDM ACTIVITY NOTATION:
–
–
–
–
Activity Number: unique number to an activity
Duration: number of “work periods” assigned
ES & EF: calculated in Forward Pass
LS & LF: calculated in Backward Pass
ACTIVITY NUMBER
ES
1
A
5
EF
LS
3
5
7
LF
DURATION
FORWARD & BACKWARD PASS
10
40
14
5
START
20
50
60
4
4
5
30
5
Legend
FINISH
Activity ID
ES
LS
EF
Duration
LF
FORWARD & BACKWARD PASS
10
1
40
5
6
19
14
5
20
6
50
9
4
START
10
60
13
20
4
5
30
5
9
24
5
Early Dates:
Legend
ESs = EFp + 1
EFs = ESs + Ds - 1
24
FINISH
Activity ID
ES
LS
EF
Duration
LF
FORWARD & BACKWARD PASS
10
1
5
40
5
6
14
19
20
12
START
Late Dates:
4
50
15
16
4
19
20
5
24
30
11
5
15
24
Legend
LFp = LSs -1
LSp = LFp – D +1
60
FINISH
Activity ID
ES
LS
EF
Duration
LF
FORWARD & BACKWARD PASS
10
1
1
5
40
5
6
5
6
19
14
19
20
6
12
START
4
9
10
15
16
4
13
20
19
20
11
9
24
5
24
5
24
24
15
Legend
LFp = LSs -1
LSp = LFp – D +1
60
30
5
Late Dates:
50
FINISH
Activity ID
ES
LS
EF
Duration
LF
What is the Scheduling Process?
Report/ Status
Scheduling
Planning
IDENTIFY MILESTONES
• Project Start
• Project Phase Start and Finishes
• Equipment and/ or Long Lead Item
Delivery
• Project Finish
DEVELOP ACTIVITY LIST
(EXAMPLE: HOUSE CONSTURCTION)
• Select Lot
• Close Financing
• Clear and Grub Lot
• Landscape
• Install Utilities
• Purchase Appliance and
Lighting Fixtures
• Construct House
• Select Contractors
• Arrange Financing
• Install Flooring
• Paint
ASSIGN DURATION TO ACTIVITES
(EXAMPLE: HOUSE CONSTRUCTION)
•
•
•
•
•
•
•
•
•
•
•
Select Lot
Clear and Grub Lot
Install Utilities
Construct House
Select Contractors
Arrange Financing
Close Financing
Landscape
Purchase Appliance/Lighting Fixtures
Install Flooring
Paint
3 days
10 days
3 days
60 days
5 days
5 days
1 day
5 days
1 day
5 days
5 days
SEQUENCE ACTIVITIES
ASK THREE QUESTIONS!
• What activity must come before this one?
• What activity must come after this one?
• What activities can I perform at the same
time?
REVIEW AND FINALIZE
•
•
•
•
Review Relationships
Review and Finalize Durations
Review Resource Usage
Review and Finalize Project End Date
– Does this achieve the desired date?
– If it does not…. What is the plan NOW?!?
SCHEDULING- CONCLUSION
• Planning and scheduling functions are
usually performed iteratively in order to
provide for accomplishing all required
tasks within the specified time frames
• Both Critical Path Methods (ADM & PDM)
create networks showing activity durations
and total time for project completion
• “Failing to Plan is Planning to Fail”, hence
“Plan the Work, Work the Plan”