Article Date: 08/08/2014

What Is the Threshold for Cumulative Impact on a Construction Project?

By Steve Rizer


How much change in a construction project is necessary before a contractor enters a cumulative impact condition? While there is no sweeping, hard-and-fast, industry-wide answer to this question, William Ibbs and other researchers have come up with a “rule of thumb” for determining when such a condition is reached -- a condition in which, as Ibbs put it during a WPL Publishing webinar he recently co-presented, “you have so many changes that you cannot really measure the full impacts of each change on a case-by-case, discrete basis.” 


Ibbs, professor and group leader in the Construction Management Program of the University of California, Berkeley’s Civil Engineering Department, reported that, as a general rule of thumb, “a 10 change amount” represents the threshold at which contractors move into cumulative impact.


Whether the true threshold is 10 percent or something else, it should be measured by change “based on the number of labor hours on the project, not the dollars on a project,” Ibbs told professionals attending “How to Identify and Quantify the Effects of Cumulative Impact,” a 90-minute interactive event that WPL held late last month. “Cumulative impact conditions are normally disruptions to the labor productivity of a project, and changes that are predicated by dollars or measured in dollars may not really address the labor aspects of a project. You could have a lot of material changes on a project that rise to the 10 percent threshold in terms of dollars, but those changes to material and product and commodities and permanent equipment do not necessarily have an impact on labor productivity.”


Ibbs co-presented the webinar with Paul Stynchcomb, an expert in Critical Path Method scheduling, construction management, contract administration, and labor productivity. Both speakers are part of The Ibbs Consulting Group, which, in addition to cumulative impact, focuses on cost accounting, construction defects, scheduling and delay analysis, and other facets of construction business.


Also during the webinar, Ibbs and Stynchcomb additionally discussed how notice of cumulative impact can be given, how it is quantified, when to quantify effects, and other topics related to cumulative impact.


To purchase a recording of the webinar, visit



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