critical path

Report
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
2
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
3
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
4
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
5
Determining the Critical Path for Project X
6
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.
7
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.
8
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.
9
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.
10
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
11
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
12
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
13
Free and Total Float or Slack for Project
X
14
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?
25
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
26
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
27
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
28
Multitasking Example
29
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
30
Example of Critical Chain Scheduling
31
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
32
PERT Formula and Example
• PERT weighted average =
optimistic time + 4X most likely time + pessimistic time
6
• Example:
PERT weighted average =
8 workdays + 4 X 10 workdays + 24 workdays
6
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
33
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
34
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
35
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)
36
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
37
Working with People Issues
• Strong leadership helps projects succeed more
than good PERT charts
• Project managers should use:
– Empowerment
– Incentives
– Discipline
– Negotiation
38
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
39
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
40
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
41

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