Assessing Accuracy of Estimated Activity Durations in Construction CPM Schedules

This paper’s fundamental question is whether construction baseline critical path method (CPM) schedules would benefit from requirements, or generally accepted industry practices, that cap maximum construction activity durations. The research conducted determined if lower estimated duration values for construction activities are more accurate on both an activity-by-activity and global basis. This question was answered by evaluating the accuracy of duration estimates by identifying and evaluating trends in 90 schedules from 45 construction projects. The data sample represented a diverse grouping of projects by markets, delivery methods, and ownership types. In all, more than 30,000 activities were evaluated to form this paper’s observations and findings. The paper has two principle findings. First, schedules that have a higher proportion of construction activities with original durations of one to five working days are more likely to have actual durations that match the baseline estimate. Schedules where at least 90% of construction activities had estimated durations of five or less days were nearly three times more likely to complete individual tasks within the planned duration than schedules where 25% or less of construction activities had durations of one to five days. Second, activities with estimated original durations of five or less days tend to have smaller duration variances than activities with longer original durations. Construction activities with durations between six to ten days had average variances that were 3.6 times larger than activities with durations of less than five days. Accordingly, this research supports the recommendation that industry scheduling and planning efforts would benefit from standards that prescribe maximum estimated durations for construction activities. This benefit would be realized in more accurate activity duration estimates both in terms of lower variances from actual durations and higher likelihood of completed within the planned duration.