Critical Path Method (CPM) | Adroit Consultants, LLC

In a previous article (Diagrams to illustrate repetitive construction activities), we identified the main diagrams that construction project practitioners use to illustrate repetitive construction activities. In that article, we described the two main classes of linear scheduling methods (LSM) and line of balance (LOB) techniques that are used in linear projects.

Below, we are going to provide a critical comparison between the critical path method (CPM) and linear scheduling method (LSM). As a deterministic network model, the CPM method uses duration estimate for project activities to determine the longest duration path for the project and to identify the earliest and latest dates for schedule activities based on the use of forward- and backward-pass network calculations, respectively. LSM schedules, however, use velocity diagrams representing each activity. The schedule format may provide the planned and actual production rates on a time-scaled format. The main differences between the CPM and LSM methods can be summarized as follows:

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	Critical Path Method (CPM)	Linear Scheduling Method (LSM)
Application	Although this method is typically used in non-linear projects, it can also be used in linear construction projects.	It is used in linear construction projects, where the majority of the work is made up of highly repetitive activities. In these projects, a set of project activities are repeated in each location for the entire length of the work. Once a project activity is started and/or ended in one location, it is repeated in another location.
Accuracy	With using forward and backward network calculations, the CPM method determines the expected project completion with accuracy.	The LSM allows for accurately planning and scheduling of project activities from the perspectives of both time and location.
Uncertainty in activity durations	With some modifications, the CPM method can change to Program Evaluation and Review Technique (PERT) which allows for randomness by introducing uncertainty to activity duration estimates (i.e., using optimistic, most likely, and pessimistic durations to calculate the expected time for schedule completion).	The current forms of LSM do not allow for randomness in activity durations.
Uncertainty in activity relationships	With some modifications, the CPM method can change to Graphical Evaluation and Review Technique (GERT) that allows for conditional and probabilistic treatment of logical relationships (i.e., depending on the outcome of the predecessor activities, succeeding activities may or may not be performed).	The current forms of LSM do not allow for conditional and probabilistic treatment of logical relationships.
Critical path	The CPM identifies the critical path based on forward and backward network calculations.	The LSM algorithm identifies the controlling activity path (CAP) which can be considered a path with the same function as the critical path in the CPM method. The LSM also identifies location criticality.
Spatial aspects	It might be inadequate for effective planning and scheduling of linear construction projects because it does not account for work locations or spatial aspects and does not effectively model project activities that are repetitively performed.	It uses velocity diagrams representing each activity, accounts for work locations or spatial aspects, and effectively models project activities that are repetitively performed.
Readability and usefulness	The CPM method becomes convoluted in complex projects because of the high number of project activities and activity dependencies. This complexity makes it difficult for practitioners to effectively use, communicate, and understand project CPM schedules in complex projects.	The LSM method is easy to understand and an effective tool to communicate the project time objectives with all team members including those individuals who lack an in-depth knowledge of project planning and scheduling.
Ease of use and development	Computer programs have significantly facilitated the use and development of CPM schedules; however, software programs have become complicated and require extensive training.	The LSM is intuitive and can easily be produced with or without the use of computer programs. However, the limited number of computerized implementation platforms restricts the use of this method in large projects.
Ease of updating	Computer programs have significantly facilitated the process of updating CPM schedules; however, updating complex CPM schedules may become challenging due to the increased number of activity, activity dependencies, activity constraints, activity calendars, and resource calendars in these schedules.	Updating an LSM schedule is typically simple and intuitive.

References:

Mirhadi M. and Terouhid, A. (2018). AACE International Recommended Practice 91R-16 (RP 91R-16): Schedule Development. AACE International (The Association for the Advancement of Cost Engineering). Retrieved from https://web.aacei.org/docs/default-source/toc/toc_91r-16.pdf?sfvrsn=2

Adroit Consultants, LLC (2018). Diagrams to illustrate repetitive construction activities. Retrieved from https://www.adroitprojectconsultants.com/2018/08/06/diagrams-to-illustrate-repetitive-construction-activities/

Tag: Critical Path Method (CPM)

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A critical comparison between CPM and LSM