Background & HistoryDeveloped in the 1950s by the US Navy Originally, the critical path method considered only logical dependencies between terminal elements Since then, it has been expanded to allow for the inclusion of resources related to each activity, through processes called activity-based resource assignments and resource leveling. Critical Path Method for the construction industry Non-computer approach John Fondahl
John FondahlStanford CE Professor Emeritus – 35 years Passed away last September 13th, 2008 US Marine Corps Sergeant in Iwo Jima His father was USMC LtCol
Co-founder of the CEM program 1961 Paper for the US Navy – “Non-Computer Approach to the Critical Path Method for the Construction Industry”
What is CPM?The Critical Path Method or Critical Path Analysis, is a mathematically based algorithm for scheduling a set of project activities It is an important tool for effective project management Commonly used with all forms of projects, including construction, software development, research projects, product development, engineering, and plant maintenance, among others Any project with interdependent activities can apply this method of scheduling
What is CPM?The essential technique for using CPM is to construct a model of the project that includes the following: A list of all activities required to complete the project (also known as Work Breakdown Structure) The time (duration)that each activity will take to completion The dependencies between the activities.
What is CPM?CPM calculatesThe longest path of planned activities to the end of the project The earliest and latest that each activity can start and finish without making the project longer Determines “critical” activities (on the longest path) Prioritize activities for the effective management and to shorten the planned critical path of a project by: Pruning critical path activities “Fast tracking” (performing more activities in parallel) “Crashing the critical path” (shortening the durations of critical path activities by adding resources)
The CPM ApproachPhase IBreak project into operations necessary for completion Determine sequential relationship of operations Every operation must have event to mark commencement – i.e. completion of preceding operation Can operations overlap?
The CPM ApproachPhase IICreate time estimates for each operation Determine earliest possible start date, earliest possible finish date , latest start & finish Determine “free float” and “total float”
Revised after completion of Phase III
The CPM ApproachPhase IIIEstablish time-cost relationship Establish scheduling variations Determine most favorable balance between time-cost
Normal Start – normal time, least cost All-Crash Start – least time, highercost
The CPM Approach
DefinitionsFloat (slack) – amount of time that a task can be delayed without causing a delay to: subsequent tasks (free float) project completion date (total float) Critical path is the sequence of activities which add up to the longest overall duration. It is the shortest time possible to complete the project.
Any delay of an activity on the critical path directly impacts the planned project completion date (there is no float on the critical path). A project can have several, parallel, near critical paths. An additional parallel path through the network with the total durations shorter than the critical path is called a sub-critical or non-critical path. Critical activity – activity with zero float Resource leveling – iterative process of assigning crews to activities in order to calculate their duration
DefinitionsActivity Identity boxEarly Start (ES) Early Finish (EF)
Activity IDLate Start (LS) Late Finish (LF)
Class ExerciseA STAR T C G H B D FActivity ID A B C D E F G H Duration 7 3 6 3 3 2 3 2 A B D,F B C E,G Dependency
Class Exercise0 A 0 7 7 7 7 7 C 6 13 13 13 13 G 3 16 16 H 16 0 B 7 3 10 10 3 3 D 3 13 13 66 E 3 16 9 2 18 18 16
3 F 11
5 2 13
Class ExerciseGantt Chart
SummaryCritical Path Analysis is an effective and powerful method of assessing: Tasks which must be carried out Where parallel activity can be carried out The shortest time in which a project can be completed Resources needed to achieve a project The sequence of activities, scheduling, and timings involved Task priorities
ReferencesFondahl, John W., “Non-Computer Approach to the Critical Path Method for the Construction Industry”, Report #9, Stanford University, 1961 Fondahl, John W. “The History of Modern Project Management – Precedence Diagramming Methods: Origins and Early Development”. Project Management Journal. Volume XVIII. No. 2. June 1987. Hendrickson, Chris & Au, Tung, Project Management for Construction, Prentice Hall, 1989 Weber, Sandra C., Scheduling Construction Projects: Principles and Practices, Prentice Hall, 2005