Functional Size Measurement and Function Point Methods

Functional Size Measurement and Function Points

Many software metrics can be valuable, but any professionally-managed software development programme should be able to monitor and record for each project the three main performance parameters of:

• Productivity (= size / effort)
• Speed of delivery (= size / elapsed time)
• Quality (= no. of defects delivered into production / size)

To arrive at these parameters, input measures of effort consumed (such as Lines of Code) are not sufficient. All three of these essential performance measures depend on having a measure of work-output that is independent of the technology used for the software. As such measures depend only on the functionality required by and delivered to the user they are known as ‘functional size measures'

To arrive at such measures, we use units of functional size known as 'function points.' This technique was pioneered by Allan Albrecht at IBM in 1978. The method developed by Albrecht, IFPUG FPA, continues to be used. The governing body is the International Function Point User Group based in the USA.

In 1999, the Common Software Metrics International Consortium released a revised functional size measurement method specifically designed for use with modern software development methods. This is the COSMIC method which can be used with MIS, real-time and embedded systems, and is also ideally suited to measuring of Agile performance and productivity.

Comparison of the most common Functional Size Methods (FSM)

Download pdf of Grant Rule's chart comparing IFPUG, NESMA, Mark II and COSMIC.

Three methods of sizing

These techniques conform to the ISO Standard for the Functional Size Measurement of Software ISO/IEC 14143:1998 and each has approval as an ISO/IEC standard.

COSMIC FP – ISO/IEC 19761
Used in a wider range of problem domains, such as real-time or infrastructure development, as well as the business/MIS domain. The method caters for software system types; avionics, telecoms, production systems and so on. It has has been specifically designed to fit with the complex multi-layered architectures and approaches to modern software development. COSMIC FP demonstrates an advance in generality, simplicity and accuracy.

MkII FPA– ISO/IEC 20968
A method designed to address the structured approaches in software design. The MKII method is equally useful in the area of object oriented design, fitting well with the use case notation. It is the preferred method of the UK government as it is easily applied to large systems.

IFPUG FPA – ISO/IEC 20926
The original method of sizing, it is currently at version 4. This method is still the most widely used and works well in the business/MIS domain. It is applicable to object oriented developments.

SMS Credentials in FPA

SMS are specialists in functional size measurement, performing FP counts, analysis, audits and health-checks. SMS credentials with respect to all methods of Function Point Analysis are second to none. SMS consultants were contributors to the COSMIC core team. Charles Symons, the current lead for international COSMIC team, is a retired director of SMS and is still available for specialist COSMIC consultancy.

Members of the SMS team have contributed significantly to the development of industry standards at both national and international levels, including participation on the Counting Practices Committees of the design authorities for IFPUG FPA, Mark II FPA and COSMIC FP Methods. This includes work with the British Standards Institute (BSI) and the International Standards Organisation (ISO). The sometime national representatives to ISO SC7 WorkGroup12 for both the United Kingdom and the Republic Of Ireland operate as SMS consultants, and several have contributed to the current work of ISO SC7 WorkGroup 6 .