Functional Size Measurement (FSM) is a technique for measuring software in terms of the functionality it delivers. The ISO/IEC standard for Functional Size Measurement 14143-1:2006 defines FSM as a means of quantifying the Functional User Requirements i.e. functions that the user has required to be delivered. (For more information on the IFPUG Method for counting function points see - Introduction to Function Point Analysis.)
Functional Size can be used for many purposes (See - Uses & Benefits of Sizing) however; it is primarily used at the planning stage for input into project resource estimation calculations for cost, effort and schedule.At the completion of a project it is used to compare performance in terms of the cost effectiveness and efficiency of the development and support teams.
FSM measures the functional requirements of the software. This means that it can be applied before development commences, rather than retrospectively as is the case with other forms of software measurement, such as counting lines of code and/or other physical objects. This capability to measure early enables accurate estimates to be made, risks to be evaluated, and project scope to be negotiated, before final commitments are made.
FSM also enables comparison of applications and projects based on their size. Productivity rates for applications of a similar attribute profile can be compared for benchmarking support ratios and improvement purposes. Productivity rates from past projects can also be used to predict effort, once a project’s Functional Size has been determined.
SCOPE supports the rules of the Functional Size Measurement method, ISO/IEC 20926:2008 standard IFPUG Unadjusted and the IFPUG Function Point Analysis Method CPM 4.3. This technique is often referred to as IFPUG Function Point Analysis or “FPA”.