Variation in construction task starting time and duration can impact overall productivity performance, yet their root causes often remain unclear. In this research, variation is defined as the difference between what was planned and what actually happened. This paper examines the variation in starting time and the duration of construction tasks while considering 50 productivity factors. The 50 factors were divided into eight categories: prerequisite work, detailed design / working method, labor force, tools and equipment, material and components, work / jobsite conditions, management / supervision / information flow, and weather or external conditions. A nationwide survey involving craft workers, foremen, and project managers was employed and serves to offer valuable insight on the causes and impacts of variation. The findings will help construction project managers and field managers identify the common risk factors, focus on the root causes of variation in planning phase, develop effective strategies to reduce variation, and improve project.