Transcript critical path

Project Time Management
Part 2
Critical Path Method (CPM)
• CPM is a network diagramming technique
used to predict total project duration
• A critical path for a project is the series of
activities that determines the earliest time by
which the project can be completed
• The critical path is the longest path through
the network diagram and has the least
amount of slack or float
• Slack or float is the amount of time an activity
may be delayed without delaying a
succeeding activity or the project finish date
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Critical Path Method (CPM)
• There are normally several tasks done in
parallel on a project
• Most projects have multiple paths on the
network diagram
• The longest path or the path containing the
critical tasks is deriving the completion dates
for a project
• You are not finished with the project until you
have completed all the tasks
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Calculating the Critical Path
• First develop a good network diagram, which
in turn requires a good activity list based on
the WBS.
• Estimate the duration of each activity
• Add the duration estimates for all activities on
each path through the network diagram
• The longest path is the critical path
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Calculating the Critical Path (Cont’d)
• What does critical path really mean?
• The critical path shows the shortest time in
which a project can be completed.
• If one or more of the activities on the critical
path takes longer than planned, the whole
project schedule will slip unless the project
manager takes corrective action
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Determining the Critical Path for Project X
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More on the Critical Path
• A project team at Apple computer put a stuffed
gorilla on the top of the cubicle of the person
currently managing critical task
• This would let others know that the person under
the pressure don’t need to be disturbed or
distracted.
• The critical path is not the one with all the critical
activities; it only accounts for time
• There can be more than one critical path if the
lengths of two or more paths are the same
• In case of more than one critical path the project
managers should keep track of all those critical paths
• The critical path can change as the project
progresses.
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How Critical Paths Change
• Suppose activities A,B,C,D,E,F and G all start and finish as
planned
• Assume that activity I runs into problems and takes more
than 4 days to complete
• This would make path C,G,I,J be longer than other paths
• Hence C,G,I,J would now be the Critical Path.
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Using Critical Path Analysis to Make
Schedule Trade-offs
• It is important to know what the critical path is
throughout the life of a project. So the project
manager can make tradeoffs
• Behind schedule tasks on the critical path need to
be addressed by answering these questions
– Should the schedule be renegotiated?
– Should more resources be allocated on other items on
the critical path to make up for lost time?
– Is it ok if the project finishes behind schedule?
• Project team can proactively manage schedule
using the critical path analysis.
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Using Critical Path Analysis to Make
Schedule Trade-offs
• Project Manager can use a technique to make
tradeoffs. This technique involves determining the
following
• Free slack or free float is the amount of time an
activity can be delayed without delaying the early
start of any immediately following activities
• Total slack or total float is the amount of time an
activity may be delayed from its early start without
delaying the planned project finish date
• Early Start Date is the earliest possible time an
activity can start based on the project network
logic.
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Using Critical Path Analysis to Make
Schedule Trade-offs
• Project Managers calculate free or total slack
by conducting a forward pass or backward
pass on the network diagram
• A forward pass through the network diagram
determines the early start and finish dates
• A backward pass determines the late start and
finish dates
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Example of Forward Pass
• Tasks D and E precedes task H
• Early start date for task H would be early finish
date of task E. As it’s greater than early finish
date of task D
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Calculating Early and Late Start and Finish Dates
• B,C is critical path with no float or slack
• Task A has 5 days of free and total float or slack
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Free and Total Float or Slack for Project
X
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How to Find the Critical Path
• General Foundry’s network with expected activity times
A
2
C
2
F
E
3
4
H
Start
2
Finish
B
3
D
4
G
5
How to Find the Critical Path
•
1.
2.
3.
4.
To find the critical path, need to determine the following quantities for
each activity in the network
Earliest start time (ES): the earliest time an activity can begin without
violation of immediate predecessor requirements
Earliest finish time (EF): the earliest time at which an activity can end
Latest start time (LS): the latest time an activity can begin without
delaying the entire project
Latest finish time (LF): the latest time an activity can end without delaying
the entire project
How to Find the Critical Path
• In the nodes, the activity time and the early and late start and finish
times are represented in the following manner
ACTIVITY
ES
LS
t
EF
LF
 Earliest times are computed as
Earliest finish time = Earliest start time
+ Expected activity time
EF = ES + t
Earliest start = Largest of the earliest finish times of
immediate predecessors
ES = Largest EF of immediate predecessors
How to Find the Critical Path
• At the start of the project we set the time to zero
• Thus ES = 0 for both A and B
A
ES = 0
t=2
EF = 0 + 2 = 2
B
ES = 0
t=3
EF = 0 + 3 = 3
Start
How to Find the Critical Path
• General Foundry’s ES and EF times
A
0
2
2
C
2
2
4
F
4
E
4
Start
B
0
Figure 13.4
3
3
D
3
4
7
3
7
4
8
H
13
G
8
5
13
2
15
Finish
How to Find the Critical Path
• Latest times are computed as
Latest start time = Latest finish time
– Expected activity time
LS = LF – t
Latest finish time = Smallest of latest start times
for following activities
LF = Smallest LS of following activities
 For activity H
LS = LF – t = 15 – 2 = 13 weeks
How to Find the Critical Path
• General Foundry’s LS and LF times
A
0
0
2
2
2
C
2
2
2
4
4
F
4
10
E
4
4
Start
B
0
1
Figure 13.5
3
3
4
D
3
4
4
7
8
3
7
13
4
8
8
H
13
13
G
8
8
5
13
13
2
15
15
Finish
How to Find the Critical Path
• Once ES, LS, EF, and LF have been determined, it is a simple matter to
find the amount of slack time that each activity has
Slack = LS – ES, or Slack = LF – EF
• From Table 13.3 we see activities A, C, E, G, and H have no slack time
• These are called critical activities and they are said to be on the critical
path
• The total project completion time is 15 weeks
• Industrial managers call this a boundary timetable
How to Find the Critical Path
• General Foundry’s schedule and slack times
ACTIVITY
EARLIEST
START,
ES
EARLIEST
FINISH,
EF
LATEST
START,
LS
LATEST
FINISH,
LF
SLACK,
LS – ES
ON CRITICAL
PATH?
A
0
2
0
2
0
Yes
B
0
3
1
4
1
No
C
2
4
2
4
0
Yes
D
3
7
4
8
1
No
E
4
8
4
8
0
Yes
F
4
7
10
13
6
No
G
8
13
8
13
0
Yes
H
13
15
13
15
0
Yes
Table 13.3
How to Find the Critical Path
• General Foundry’s critical path
A
0
0
2
2
2
C
2
2
2
4
4
F
4
10
E
4
4
Start
B
0
1
Figure 13.6
3
3
4
D
3
4
4
7
8
3
7
13
4
8
8
H
13
13
G
8
8
5
13
13
2
15
15
Finish
Using the Critical Path to Shorten a
Project Schedule
• Three main techniques for shortening schedules
for activities on the critical path.
– Shortening durations of critical activities/tasks by
adding more resources or changing their scope
– Crashing is a technique used to decrease the duration
of the project by assigning additional resources to
tasks and decreasing the duration required for those
tasks. Main disadvantage?
– Fast tracking activities by doing them in parallel or
overlapping them. Main disadvantage?
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Importance of Updating Critical Path
Data
• It is important to update project schedule
information to meet time goals for a project
• The critical path may change as you enter
actual start and finish dates
• If you know the project completion date will
slip, negotiate with the project sponsor
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Critical Chain Scheduling
• Critical chain scheduling
– A method of scheduling that considers limited
resources when creating a project schedule and
includes buffers to protect the project completion
date
• Uses the Theory of Constraints (TOC)
– This theory says that at any given time the project has
an aspect or constraint which limits its ability to
achieve more of its goals.
– For the system to attain any significant improvements
the constraint might be identified and the whole
system must be managed with it in mind
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Critical Chain Scheduling
• The important thing is availability of scarce
resources like people, software or hardware.
• Attempts to minimize multitasking
– When a resource works on more than one task at
a time
– Multitasking can delay task completion
– It can some times increase total duration like
wasted setup time
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Multitasking Example
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Buffers and Critical Chain
• A buffer is additional time to complete a task
• Murphy’s Law states that if something can go
wrong, it will, like fears about estimates being
reduced from sponsor side.
• Parkinson’s Law states that work expands to fill the
time allowed
• In traditional estimates, people often add a buffer
to each task and use it if it’s needed or not
• Critical chain scheduling removes buffers from
individual tasks and instead creates:
– Project buffers or additional time added before the
project’s due date
– Feeding buffers or additional time added before tasks
on the critical path
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Example of Critical Chain Scheduling
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Program Evaluation and Review
Technique (PERT)
• PERT is a network analysis technique used to
estimate project duration when there is a high
degree of uncertainty about the individual
activity duration estimates
• PERT uses probabilistic time estimates
– Duration estimates based on using optimistic, most
likely, and pessimistic estimates of activity durations,
or a three-point estimate
• The main disadvantage is more work required
than CPM
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PERT Formula and Example
• PERT weighted average =
optimistic time + 4X most likely time + pessimistic time
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• Example:
PERT weighted average =
8 workdays + 4 X 10 workdays + 24 workdays
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where optimistic time = 8 days
most likely time = 10 days, and
pessimistic time = 24 days
= 12 days
Therefore, you’d use 12 days on the network diagram instead of
10 when using PERT for the above example
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Schedule Control
• The final process in project time management is
schedule control
• Like scope control the schedule control is a
portion of integrated change control process
under the project integration management
• The goal of schedule control is to know the status
of the schedule, influence the factors causing
schedule changes, determine schedule changes
and manage changes when they occur
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Schedule Control Suggestions
• Perform reality checks on schedules
• Allow for contingencies
• Don’t plan for everyone to work at 100%
capacity all the time
• Hold progress meetings with stakeholders and
be clear and honest in communicating
schedule issues
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Controlling the Schedule
• It is important to address conflicts and allow
management insight if any thing gets out of hand
• Project managers must use discipline to control
schedule like setting firm dates for key milestones
and emphasizing the need to follow them
• Tools and techniques include:
– Progress reports
– A schedule change control system
– Project management software, including schedule
comparison charts like the tracking Gantt chart
– Variance analysis, such as analyzing float or slack
– Performance management, such as earned value (Chapter
7)
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Reality Checks on Scheduling
• First review the draft schedule or estimated
completion date in the project charter
• Prepare a more detailed schedule with the
project team
• Make sure the schedule is realistic and
followed
• Alert top management well in advance if there
are schedule problems
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Working with People Issues
• Strong leadership helps projects succeed more
than good PERT charts
• Project managers should use:
– Empowerment
– Incentives
– Discipline
– Negotiation
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Using Software to Assist in Time
Management
• Software for facilitating communications helps
people exchange schedule-related information
in the form of Network diagrams with critical
paths information, Gantt charts etc
• Decision support systems help analyze tradeoffs that can be made
• Project management software can help in
various time management areas
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Words of Caution on Using Project
Management Software
• Many people misuse project management
software because they don’t understand
important concepts behind creating a network
diagram, evaluating critical path or setting a
schedule baseline
• You must enter dependencies to have dates
adjust automatically and to determine the critical
path
• You must enter actual schedule information to
compare planned and actual progress
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Assignment # 1
• Make a team of up to 3 people
• Select a Software Project of your own choice
• Develop Project Charter of the selected
project
• Deadline of this deliverable is 22-10-2013
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