Conceptual Association of Functional Size Measurement Methods Software Estimation

Although functional size measurement (FSM) methods give roughly similar results, they’ve been designed neither to measure the same attribute nor to use the same rating scale. Consequently, when measured using different methods, a software product has different functional sizes. Each method has its own definition of functionality and uses different counting schemas for different func­ tional user requirements (FUR) entities. Moreover, FSM methods use their own underlying software engineering concepts rather than those of a particular software development methodology.2 They use different abstractions during their measurement processes or view the same concepts from different perspectives.3,4 In practice, these differences cause difficulties. In many cases, software organizations must change the FSM method used or convert one FSM method’s size measures to that of another. Several things might influence the need for this, including the acquirers’ requirements, a specific method’s suitability for a new implementation domain, or the need to use benchmark sets established in specific implementation domains. Because measurement processes, rules, and counting schemas aren’t the same, changing existing methodology is difficult for organizations. An inability to use the established historical data in a new methodology makes the change even more challenging. In addition, the lack of a well-established conversion formula creates difficulties for developing and using benchmark sets. To overcome these difficulties, we developed the Unified Model (UM) to measure functional size via three FSM methods simultaneously: International Function Point Users Group (IFPUG) function point analysis (FPA),5 Mark II (MkII) FPA,6 and Common Software Measurement International Consortium (Cosmic) FP.7 With some limitations, UM lets us convert one method’s functional sizes to those of another method, thus addressing both the need for conversion and the need to use different methods in a single project or organization. To develop UM, we first examined the similarities and differences between the three FSM methods and defined a unified terminology for the common measurement concepts they use. F unctional size determines how much functionality software provides by measuring the aggregate amount of its cohesive execution sequences. Alan Albrecht first introduced the concept in 1979.1 Since he originally described the func­ tion point analysis (FPA) method, researchers and practitioners have developed variations of functional size metrics and methods (see the “Related Research in Functional Size Measurement” sidebar). The authors discuss the conceptual similarities and differences between functional size measurement methods and introduce a model for unification. Onur Demirors, Middle East Technical University