Post on 15-Jan-2016
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LECTURE 7
Calculating the Critical Path
Introduction
Critical Path:
• The path through the network that takes the longest total time, and
• Determines the earliest possible time the project can be completed.
All activities on this path have zero float; the early and late start (and early and late finish) are the same.
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What the Critical Path Calculates
Earliest Start (ES) = the earliest time at which the activity can begin.
• Calculated by a forward pass through the network diagram,
• Calculated on the basis of the project’s start time and the duration the preceding activity.
Earliest Finish (EF) = the earliest time by which the activity can be completed
• Calculated by adding the activity’s duration estimate to the earliest start time.
EF= ES + Duration estimate
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Latest Finish (LF) = the latest time that the activity can end and still allow the project to be completed on time.
• Calculated by a backward pass through the network diagram
• Calculated on the basis of the project’s completion time and the duration for the succeeding activities. Generally, the latest finish time of the activity is equal to the earliest of all the lastest start times of all activities emerging directly from that activity.
Latest Start (LS) = the latest time that the activity can begin and still allow the project to be completed on time.
• Calculated by subtracting the activity’s duration from the activity’s latest finish time
LS= LF – Duration estimate
What the Critical Path Calculates
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Total Slack: The time difference between the scheduled completion date and required date to meet critical path
Total Slack = LF- EF or
Total Slack = LS - ES
• If Total Slack >0: some activities can be delayed without jeopardizing completion of the project by its required completion time
• If Total Slack <0: some activities need to be accelerated in order to complete the project by its required completion time
Total Slack / Float
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Slack Identification
C (8,10)
2 (15,17)
EARLIEST START TIME (ES)
EARLIEST FINISH TIME (EF)
LATEST FINISH TIME (LF)
LATEST START TIME (LS)
ACTIVITY
TIME
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Types Of Slack
[ 20, 26 ]
[ 24, 30 ]
[ 30, 36 ]
[ 24, 30 ]
POSITIVE SLACK NEGATIVE SLACK
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Negative Slack
FORWARD PASS
BACKWARD PASS
CUSTOMER’S CUSTOMER’SSTART DATE FINISH DATE
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2
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1
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Slack Identification
Activity Description
ESEarliest Start
Duration Estimate
Person Responsible
Activitiy Number
EFEarliest Finish
LSLatest Start
LFLatest Finish
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Picnic at the Lake - Activities
Activity Description
Person Responsible
Duration (mins.) Preceding activity
1. Decide on trip Husband & Wife 2 none
2. Get money from bank
Wife 12 1
3. Boil eggs Husband 10 1
4. Make egg sandwiches
Husband 10 3
5. Load car Husband & Wife 5 2,4
6. Drive to lake Husband & Wife 30 5
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Picnic at the Lake
Decide on tripMin: 2 Float:Early Start:Early Finish:Late Start:Late Finish:
Get money from bankMin: 12 Float:Early Start:Early Finish:Late Start:Late Finish:
Boil eggsMin: 10 Float:Early Start:Early Finish:Late Start:Late Finish:
Make egg sandwichesMin: 10 Float:Early Start:Early Finish:Late Start:Late Finish:
Load carMin: 5 Float:Early Start:Early Finish:Late Start:Late Finish:
Drive to lakeMin: 30 Float:Early Start:Early Finish:Late Start:Late Finish:
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Classroom Activity 10a
Picnic at the lake – create planned and preceding activities network diagram.
Determine critical path.
Add early start, late start, early finish, and late finish to each activity.
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Questions?