AbstractCumulative impact of changes can be a factor impacting labor productivity. It is considered as the ripple effect of multiple changes. Because individual change orders for scope changes normally cover their direct effects, it is difficult to accurately predict the cumulative impacts and adequately capture them in the forward priced change orders. Though certain research on the timing of changes was done through statistical analyses, the timing of changes and the severity of their cumulative impact have not been rigorously defined mathematically. This paper presents a rigorous mathematical model defining the timing of changes and the severity of their cumulative impact, which is applied to the published data. Due to limited size of the published data, the numerical research remains conceptual. Through numerical analysis, this paper investigates how changes affected the labor productivity for the base scope work before, during, and after the performance of changed work, under the premise that cumulative impact of changes negatively impacted labor productivity. For various sections of a specific project, productivity losses due to cumulative impact of changes need to be assessed on a case-by-case basis in the light of the timing of changes.