Sizing the Entire Development Process-Functional and Non-functional Perspectives

The single factor that best correlates with software development effort is size. This knowledge has led to the development of several size measurement methods, both 'physical' and 'logical'. Although the functional size accounts for most of the effort variation, a large part of that variation is still unexplained by the functional size only. The lack of a standard way of dealing with non-functional factors led IFPUG to the idea of 'non-functional size', a proposed measure of the impact of all non-functional factors over a software development project.