projects: critical paths
DESCRIPTION
Projects: Critical Paths. Dr. Ron Lembke Operations Management. PERT & CPM. Network techniques Developed in 1950’s CPM by DuPont for chemical plants PERT by U.S. Navy for Polaris missile Consider precedence relationships & interdependencies Each uses a different estimate of activity times. - PowerPoint PPT PresentationTRANSCRIPT
Projects:Critical Paths
Dr. Ron LembkeOperations Management
PERT & CPM• Network techniques• Developed in 1950’s
• CPM by DuPont for chemical plants• PERT by U.S. Navy for Polaris
missile• Consider precedence relationships
& interdependencies• Each uses a different estimate of
activity times
• Completion date?• On schedule? Within budget?• Probability of completing by ...?• Critical activities?• Enough resources available?• How can the project be finished early at
the least cost?
Questions Answered by PERT & CPM
PERT & CPM Steps
• Identify activities• Determine sequence• Create network• Determine activity times• Find critical path
• Earliest & latest start times • Earliest & latest finish times • Slack
Activity on Node (AoN)
24? Years
Enroll Receive diploma
Project: Obtain a college degree (B.S.)
1 month
Attend class, study etc.
11 day
3
Activity on Arc (AoA)
4,5 ? Years
EnrollReceive diploma
Project: Obtain a college degree (B.S.)
1 month
Attend class, study,
etc.1
1 day2 3 4
AoA Nodes have meaning
GraduatingSeniorApplicant
Project: Obtain a college degree (B.S.)
1
Alum
2 3 4
Student
Liberal Arts Sidebar
• Alum = ? Alumnus
Alumna
Alumni
Alumnae
Alumni
Network Example
You’re a project manager for Bechtel. Construct the network.
Activity PredecessorsA --B AC AD BE BF CG DH E, F
Network Example - AON
A
C
E
F
BD
G
H
Z
Network Example - AOA
2
4
51
3 6 8
7 9A
C F
EBD
H
G
AOA Diagrams
2 31A
C
BD
A precedes B and C, B and C precede D
2 41A C
B
D
3
5
4
Add a phantom arc for clarity.
Critical Path Analysis• Provides activity information
• Earliest (ES) & latest (LS) start• Earliest (EF) & latest (LF) finish• Slack (S): Allowable delay
• Identifies critical path• Longest path in network• Shortest time project can be
completed• Any delay on activities delays project• Activities have 0 slack
Critical Path Analysis Example
Event ID Pred. Description Time
(Wks) A None Prepare Site 1 B A Pour fdn. & frame 6 C B Buy shrubs etc. 3 D B Roof 2 E D Do interior work 3 F C Landscape 4 G E,F Move In 1
Network Solution
A
EDB
C F
G
1
6 2 3
1
43
Earliest Start & Finish Steps
• Begin at starting event & work forward• ES = 0 for starting activities
• ES is earliest start• EF = ES + Activity time
• EF is earliest finish• ES = Maximum EF of all predecessors for
non-starting activities
Activity ES EF LS LF SlackA 0 1BCDEF
Activity AEarliest Start Solution
For starting activities, ES = 0.
AEDB
C F
G1
6 2 31
43
Activity ES EF LS LF Slack A 0 1 B 1 7 C 1 4 D 7 9 E 9 12 F 4 8 G 12 13
Earliest Start Solution
AEDB
C F
G1
6 2 31
43
Latest Start & Finish Steps
• Begin at ending event & work backward• LF = Maximum EF for ending activities
• LF is latest finish; EF is earliest finish• LS = LF - Activity time
• LS is latest start• LF = Minimum LS of all successors for
non-ending activities
Activity ES EF LS LF SlackA 0 1B 1 7C 1 4D 7 9E 9 12F 4 8G 12 13 13
Earliest Start Solution
AEDB
C FG
16 2 3
1
43
Activity ES EF LS LF Slack A 0 1 0 1 B 1 7 1 7 C 1 4 5 8 D 7 9 7 9 E 9 12 9 12 F 4 8 8 12 G 12 13 12 13
Latest Finish Solution
AEDB
C F
G
1
6 2 31
43
Activity ES EF LS LF Slack A 0 1 0 1 0 B 1 7 1 7 0 C 1 4 5 8 4 D 7 9 7 9 0 E 9 12 9 12 0 F 4 8 8 12 4 G 12 13 12 13 0
Compute Slack
Critical Path
A
EDB
C F
G
1
6 2 3
1
43
New notation
• Compute ES, EF for each activity, Left to Right
• Compute, LF, LS, Right to Left
C 7LS LF
ES EF
Exhibit 6
A 21
E 5D 2B 5
C 7 F 8
G 2
Exhibit 6
A 21
E 5D 2B 5
C 7 F 8
G 2
21 28 28 36
36 38
28 3326 2821 26
0 21
F cannot start until C and D are done.G cannot start until both E and F are done.
Exhibit 6
A 21
E 5D 2B 5
C 7 F 8
G 2
21 26
0 21
26 28 31 36
36 38
21 28 28 36
21 28 28 36
36 38
28 3326 2821 26
0 21
E just has to be done in time for G to start at 36, so it has slack.D has to be done in time for F to go at 28, so it has no slack.
Gantt Chart - ES
0 5 10 15 20 25 30 35 40
A
B
C
D
E
F
G
Solved Problem 2
A 1
B 4
C 3
D 7
E 6
F 2
H 9
I 4
G 7
Solved Problem 2
A 10 1
0 1
B 41 5
1 5
C 36 9
1 4
D 72 9
1 8
E 65 11
5 11
F 29 11
8 10
H 99 18
8 17
I 418 22
18 22
G 711 18
11 18
Summary
• Activity on Node representation• Calculated
– ES, EF for all activities – LS, LF for all activities (working backwards)– Slack for each activity
• Identified critical path(s)