a systematic method to determine the critical path

The Critical Path

Method

This presentation is part ofAce the PMP Exam

A systematic method for determining the critical path

The critical path is the longest path in a network diagram, and therefore determines the shortest time that it will take to complete a project.

This systematic way of determining the critical path uses the techniques of forward pass and backward pass.

You use a forward pass to calculate the Early Start and Early Finish times and a backward pass to calculate the Late Start and Late Finish times, and therefore calculate the float. Let’s look at what these mean.

•Early Start — the earliest time that the activity can logically start

•Early Finish — the earliest time that the activity can logically finish

•Late Start — the latest time the activity can logically start without delaying the project or a milestone

•Late Finish — the latest time the activity can logically finish without delaying the project or a milestone

•Float — the amount of time an activity can be delayed without affecting the project schedule

The Forward Pass

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StartStart EndEnd

The Forward Pass

We will use the forward pass to determine the Early Start and Early Finish values of all the activities.

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StartStart EndEnd

The Early Finish is The Early Finish is the Early Start plus the Early Start plus the duration of the the duration of the

activityactivity

The Early Start is the The Early Start is the value from the value from the StartStart

MilestoneMilestone

We will use the forward pass to determine the Early Start and Early Finish values of all the activities.

Activity A

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StartStart EndEnd

activityactivity

The Early Start is The Early Start is the value from the value from

the the StartStart MilestoneMilestone

activityactivity

The Early Start is the The Early Start is the value from the value from the StartStart

MilestoneMilestone

Activity B

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StartStart EndEndThe Early Start is The Early Start is the value from the value from

activityactivity

Activity C

Activity B

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StartStart EndEnd

activityactivity

The Early Start is The Early Start is the Early Finish the Early Finish

value from value from Activity AActivity A

activityactivity

Activity D

Activity C

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2 5 F3F3

8 10StartStart EndEnd

The Early Finish is the The Early Finish is the Early Start plus the Early Start plus the

duration of the activityduration of the activity

value from value from Activity Activity B B (not (not Activity CActivity C) ) as this is the later as this is the later Early Finish time.Early Finish time.

activityactivity

value from value from Activity AActivity A

Activity DActivity E

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2 5 F3F3

The Early Start is the Early The Early Start is the Early Finish value from Finish value from Activity B Activity B (not (not Activity DActivity D) as this is the ) as this is the

later Early Finish time.later Early Finish time.

value from value from Activity Activity B B (not (not Activity CActivity C) ) as this is the later as this is the later Early Finish time.Early Finish time.

Activity E

Activity F

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2 5 F3F3

End MilestoneActivity F

The Early Start is the Early The Early Start is the Early Finish value from Finish value from Activity B Activity B (not (not Activity DActivity D) as this is the ) as this is the

later Early Finish time.later Early Finish time.

The value for the The value for the End End milestone is the milestone is the Early Finish value from Early Finish value from Activity F Activity F (not (not

Activity EActivity E) as this is the later Early ) as this is the later Early Finish time.Finish time.

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2 5 F3F3

That completes the forward pass

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End Milestone

The value for the The value for the End End milestone is the milestone is the Early Finish value from Early Finish value from Activity F Activity F (not (not

Activity EActivity E) as this is the later Early ) as this is the later Early Finish time.Finish time.

The Backward Pass

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We will use the backward pass to determine the Late Start and Late Finish values of all the activities.

The Backward Pass

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The Late Start is the Late The Late Start is the Late Finish minus the duration of Finish minus the duration of

the activitythe activity

The Late Finish is the The Late Finish is the value from the value from the EndEnd

MilestoneMilestone

We will use the backward pass to determine the Late Start and Late Finish values of all the activities.

Activity F

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0 2 D3D3

2 5 F3F3

StartStart EndEndThe Late Start is the The Late Start is the Late Finish minus the Late Finish minus the

The Late Finish The Late Finish is the value is the value

from the from the EndEnd MilestoneMilestone

The Late Finish is the The Late Finish is the value from the value from the EndEnd

MilestoneMilestone

Activity EActivity F

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0 2 D3D3

StartStart EndEnd

The Late Finish is the The Late Finish is the Late Start value from Late Start value from

Activity FActivity F

The Late Start is the The Late Start is the Late Finish minus the Late Finish minus the

The Late Finish The Late Finish is the value is the value

from the from the EndEnd MilestoneMilestone

Activity DActivity E

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StartStart EndEndThe Late Start is The Late Start is the Late Finish the Late Finish

minus the minus the duration of the duration of the

activityactivity

Activity EActivity E

Activity FActivity F

Activity CActivity D

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StartStart EndEnd

The Late Start is the The Late Start is the Late Finish minus Late Finish minus the duration of the the duration of the

activityactivity The Late Finish is the Late Start The Late Finish is the Late Start value from value from Activity F Activity F (not (not

Activity EActivity E) as this is the earlier ) as this is the earlier Late Start time.Late Start time.

The Late Start is The Late Start is the Late Finish the Late Finish

minus the minus the duration of the duration of the

activityactivity

Activity EActivity E

Activity B

Activity C

Activity B

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StartStart EndEnd

activityactivity

The Late Finish is The Late Finish is the Late Start the Late Start

value from value from Activity DActivity D

Activity A

activityactivity The Late Finish is the Late Start The Late Finish is the Late Start value from value from Activity F Activity F (not (not

Activity EActivity E) as this is the earlier ) as this is the earlier Late Start time.Late Start time.

That completes the backward pass

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StartStart EndEnd

Touch the screen to continueEnd

Activity A

activityactivity

The Late Finish is The Late Finish is the Late Start the Late Start

value from value from Activity DActivity D

If an activity can be delayed it is said to have float (or slack). To calculate float you subtract the Early Start from the Late Start (or the Early Finish from the Late Finish, it amounts to the same thing).

The amount of float is indicated as a number above the box.

Calculating float

Here is the network diagram with the float indicated above each activity

Calculating float

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StartStart EndEnd

The critical path is the path through the activities that have no float, in this case activities B and F.

The Critical Path

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StartStart EndEnd

That was a systematic method of determining the critical path in a network diagram using the forward pass and the backward pass.

For a simple way of determining the critical path, useful for quickly answering some questions on the PMP® exam, see A Simple Method of Determining the Critical Path video in this series.

The Critical Path

Method

This presentation is part ofAce the PMP Exam

a systematic method to determine the critical path

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