Functional Sizing Methods
The ISBSG Repository contains projects that have been sized using four types of functional sizing methods:
The majority of the projects in the ISBSG repository, that state the counting methods used, are IFPUG/NESMA. There are a growing number of COSMIC_FFP sized projects. Each of the other listed methods have more than thirty projects in the Repository. The remaining projects have either not declared the counting method used or have employed other less common methods, for example: Feature Points. Some projects also list a lines-of-code count.
If you are unable to complete a function point count, the functional size of software can be estimated.
How accurate are the FSM methods? For all current FSM methods two trained counters will achieve size figures of +/- 10 % if the user requirements are known and well specified. This is based on tests performed during more than 150 software estimating training courses. (Pekka Forselius of FiSMA ran the courses and the tests referred to). Other tests proved, that if ten project managers from different business areas try to estimate project effort without a systematic approach, including FSM methods, the ratio between the smallest and biggest estimate is 1 to 6, the worst as high as 1 to 12. The Practical Project Estimation 2nd edition book covers functional sizing and functional size estimation.
If a size measurement method claims to be a Functional Size Measurement Method, it should conform with the ISO/IEC 14143-1 international standard. At least 5 different methods, most of them widely used, can be called FSM methods. They are:
· COSMIC-FFP Functional size measurement method v 2.1 [10]
· IFPUG 4.1 Unadjusted [11]
· Mk II Function Point Analysis 1.3.1 Unadjusted [12]
· Nesma FPA method 2.1 Unadjusted [13]
· FiSMA FSM 1.1 [3]
All five methods follow the definitions and concepts of the international FSM standard. Any of these methods can be used for software size measurement, though they all have different size units. ISO has published other standards and technical reports in its 14143-series, which should help anyone selecting FSM method. The last part of this ISO series will be a “Guide for use of FSM standards”. However, the guide will neither explain differences between known methods, nor recommend any certain method to any functional domain or business area.