Critical Path Method in PMP Exam

Critical Path Method (CPM)

Critical Path is the LONGEST duration path through a network diagram and determines the shortest or earliest time to complete the project.

The Critical path is calculated by Adding up the duration's of activities of each path in a network diagram. The longest path is the critical path (The longest path duration = The shortest time

a project takes to finish all its activities).

This technique calculates theoretical EARLY start & Finish dates( ES & EF ) and LATE Start & Finish ( LS & LF) dates ,for all activities Regardless for any resource limitations.

Calculations of ES,EF : performing a forward pass analysis through the schedule network [from the beginning of the project to the task and following the dependencies in the network diagram].

Calculations of LS, LF: performing a backward pass analysis through the schedule network. [from the end of the project to the task and following the dependencies in the network diagram].

Most Project Management software's calculate ES EF LS LF for you, but you must be able to calculate

them manually on PMP exam.

The resulting dates ( ES,EF,LS,LF) are not necessarily the schedule

network; rather, they indicate the followings:

- TIME PERIODS within which the schedule activities could be scheduled.

-  Given activity duration

- Logical relationships.

- LEAD & LAG and other known constraints.

- Calculating ES,EF,LS,LF may be affected by the activity Total Float.

Total Float (slack): The amount of time that a schedule activity may be delayed from its early start date without delaying the project completion date or violating a schedule constraint.

Total Float is termed as schedule flexibility and is measured by the

positive difference between Early and Late dates (LF-EF or LS-ES).

•  Total float may be Positive(we can wait), Negative(We need to speed up the Work on this activity) or Zero(No any delay).
•   The “critical path” has either Zero or Negative total float. But critical path is normally characterized by Zero total float on the critical path.
•   Adjustments to activity duration, logical relationships, leads and lags or other schedule constraints may be necessary to produce network paths with a zero or positive total float.

􀂉 The activities in the critical path are called “Critical Activities” and almost always have no slack (Slack=0)
􀂉 The total float provides schedule flexibility.

Once the total float for a network path has been calculated then the Free float can also be determined.

􀂃􀂃 Free Float (Slack): The amount of time that an activity can be delayed without delaying the early start date of any immediate successor activity within the network path.
􀂃􀂃 Project Float (Slack): The amount of time a project can be delayed without delaying the externally imposed project completion required by the customer, sponsor or management.
􀂃􀂃 Near Critical Path: a Path with low float. A network diagram can have

multiple near critical paths.

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Float is extremely useful for:

– The project manager: Better allocation of rescourses among other areas of the project.

– Team members: To juggle multiple projects  
􀂃􀂃 The critical Path Method technique has the following benefits:
􀀹􀀹 It proves the project time estimate.
􀀹􀀹 It helps project compression.
􀀹􀀹 It indicates where to focus and what to monitor.
􀀹􀀹 It indicates what could be delayed.
􀀹􀀹 It Shows what requires immediate action.

􀀹􀀹 PM should monitor & control the Near–Critical Path as well.

Example I:

- In order to solve such questions which appear always in PMP exam, first you have to determine the possible paths exist in the chart.

(Start) - A-B-C-D-E-K - (End)

(Start) - A-B-F-G-H-J-K - (End)

(Start) - A-B-I-J-K - (End)

- Second step is to determine the length of each path based on the duration gived in the chart for each activity:

(Start) - A-B-C-D-E-K - (End)  *31 Days*

(Start) - A-B-F-G-H-J-K - (End) *43 Days*

(Start) - A-B-I-J-K - (End) *33 Days*

- So based on the definition of the Critical Path which is the LONGEST duration path through a network diagram, the critical path will be the second path : (Start) - A-B-F-G-H-J-K - (End) 


Example II:

- First Step: draw the network diagram :

- Second Step: determine the Critical path:

                        

- Third Step: determine the ES, EF, LS & LF for each activity:

- Now it is easily to calculate the float for each activity as the ES, EF and LS, LF are known for all activities.