This paper will define two factors which represent different sources of dimensional variation in complex assemblies. These two factors distinguish build variation by design from variation associated with manufacturing and assembly. The relationship of these two factors to the process capability (Cp) is defined, and their application to a typical problem illustrated.Utilizing Variation Simulation Analysis (VSA®) (Appendix) to predict build variation under ideal design assumptions, Cp may be expressed as the product of Design Capability (Cd ) and Process Deviation (Dp), using the relationship of Eq (1). Design capability (Cd) evaluates the design (component tolerances and assembly operations by design), while process deviation (Dp) measures the ability of manufacturing and assembly operations to achieve results equivalent to an ideal process.The benefits of using Cd & Dp during the design and manufacturing process are included, with emphasis on improving communication within the organization and quantitatively defining sources of build variation. Cd & Dp provide the organization with additional information which can be utilized during process analysis, process control and process improvement activities.