Critical Path Method

Acronym: 
CPM
Also Known As: 
Activity Network Diagram
Critical Path Analysis
Description: 

The critical path method (CPM) allows you to calculate the "critical path" of a project by showing the necessary order of tasks, the ideal project schedule, and possible problems with resources and scheduling and their corresponding solutions. It highlights the crucial stages where delays can hinder the project or where extra resources can accelerate the project.

Uses: 

For project scheduling.

When you are aware of the steps of a project or process, their order, and duration.

When scheduling and keeping track of tasks in a multifaceted project or process with tasks and resources that are interrelated.

When working on a project with a critical schedule, where finishing late results in serious consequences or finishing early is particularly advantageous.

How do I use this tool?: 

1. LIST ALL NECESSARY TASKS for the project or process under analysis.

2. DETERMINE CORRECT SEQUENCE OF TASKS:
a. Which tasks need to be completed before the next one can begin?
b. Which tasks can be worked on concurrently?
c. Which tasks should occur immediately after another is completed?

3. DIAGRAM THE NETWORK OF TASKS; time should flow left to right.

4. Between each two tasks, DRAW CIRCLES FOR "EVENTS." Where events signal the beginning or end of a task. The events are nodes separating tasks.

5. LOOK FOR THREE COMMON PROBLEM SITUATIONS and redraw them using "dummies" or extra events. A dummy is a dashed-line arrow separating tasks that, without the dummy, would start and stop with the same events. Note that dummies are not real tasks.

6. LABEL ALL EVENTS, numbered, in sequence.

7. DETERMINE TASK TIMES.

8. DETERMINE THE "CRITICAL PATH," which is path that is longest from beginning to end.

9. CALCULATE the earliest start (ES), earliest finish (EF), latest start (LS), and latest finish (LF) times for each event.

10. CALCULATE SLACK TIMES for each individual task as well as the project as a whole. Total slack is the time a task could be delayed without compromising the overall project schedule.
Total slack = LS - ES = LF - EF.
Free slack is the time a task could be postponed without affecting the early start of any job following it.

Expertise Required: 
Users of this tool usually have some training or experience in its use.
Recommended Supplies/Materials: 
Detailed knowledge of project being planned and all its stages
Advantages: 

Simple to use and understand.

More thorough analysis than a Gantt chart.

Disadvantages: 

Ignores time variations that can have significant impact on completion time of a complex project

Requires detailed knowledge of each project stage, including reasonable time estimates.

The entire network needs to be recalculated when critical path tasks are shortened.

Where can I go to learn more?: 

American Society for Quality. Seven new management and planning tools: arrow diagram. 2009 [cited 2009 July 23]; Available from: http://www.asq.org/learn-about-quality/new-management-planning-tools/overview/arrow-diagram.html

Bauer J, Duffy G, Westcott R. Improvement tools. The quality improvement handbook. 2nd ed. Milwaukee, WI: ASQ Quality Press; 2006. p. 109-48.

Last Modified: May 2013