Project Time Management Part 2. Critical Path Method (CPM) CPM is a network diagramming technique used to predict total project duration A 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. 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.

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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

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• B,C is critical path with no float or slack• Task A has 5 days of free and total float or slack

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

H 2E 4

B 3 D 4 G 5

F 3

Start Finish

How to Find the Critical Path• To find the critical path, need to determine the following quantities for

each activity in the network1.1. Earliest start timeEarliest start time (ESES): the earliest time an activity can begin without

violation of immediate predecessor requirements2.2. Earliest finish timeEarliest finish time (EFEF): the earliest time at which an activity can end3.3. Latest start timeLatest start time (LSLS): the latest time an activity can begin without

delaying the entire project4.4. Latest finish timeLatest finish time (LFLF): 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 tES EFLS LF

Earliest times are computed as

Earliest finish time =Earliest start time + Expected activity timeEF = ES + t

Earliest start = Largest of the earliest finish times ofimmediate 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

Start

A t = 2ES = 0 EF = 0 + 2 = 2

B t = 3ES = 0 EF = 0 + 3 = 3

How to Find the Critical Path

• General Foundry’s ES and EF times

A 20 2

C 22 4

H 213 15

E 44 8

B 30 3

D 43 7

G 58 13

F 34 7

Start Finish

Figure 13.4

How to Find the Critical Path• Latest times are computed as

Latest start time =Latest finish time – Expected activity timeLS = LF – t

Latest finish time = Smallest of latest start timesfor 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 20 20 2

C 22 42 4

H 213 1513 15

E 44 84 8

B 30 31 4

D 43 74 8

G 58 138 13

F 34 7

10 13

Start Finish

Figure 13.5

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 timeslack 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 activitiescritical activities and they are said to be on the critical critical

pathpath• 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 20 20 2

C 22 42 4

H 213 1513 15

E 44 84 8

B 30 31 4

D 43 74 8

G 58 138 13

F 34 7

10 13

Start Finish

Figure 13.6

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 = 12 days

6where optimistic time = 8 daysmost likely time = 10 days, andpessimistic time = 24 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)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

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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 trade-offs 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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