Estimation Techniques - Function Points


A Function Point (FP) is a unit of measurement to express the amount of business functionality, an information system (as a product) provides to a user. FPs measure software size. They are widely accepted as an industry standard for functional sizing.

For sizing software based on FP, several recognized standards and/or public specifications have come into existence. As of 2013, these are −

ISO Standards

  • COSMIC − ISO/IEC 19761:2011 Software engineering. A functional size measurement method.

  • FiSMA − ISO/IEC 29881:2008 Information technology - Software and systems engineering - FiSMA 1.1 functional size measurement method.

  • IFPUG − ISO/IEC 20926:2009 Software and systems engineering - Software measurement - IFPUG functional size measurement method.

  • Mark-II − ISO/IEC 20968:2002 Software engineering - Ml II Function Point Analysis - Counting Practices Manual.

  • NESMA − ISO/IEC 24570:2005 Software engineering - NESMA function size measurement method version 2.1 - Definitions and counting guidelines for the application of Function Point Analysis.

Object Management Group Specification for Automated Function Point

Object Management Group (OMG), an open membership and not-for-profit computer industry standards consortium, has adopted the Automated Function Point (AFP) specification led by the Consortium for IT Software Quality. It provides a standard for automating FP counting according to the guidelines of the International Function Point User Group (IFPUG).

Function Point Analysis (FPA) technique quantifies the functions contained within software in terms that are meaningful to the software users. FPs consider the number of functions being developed based on the requirements specification.

Function Points (FP) Counting is governed by a standard set of rules, processes and guidelines as defined by the International Function Point Users Group (IFPUG). These are published in Counting Practices Manual (CPM).

History of Function Point Analysis

The concept of Function Points was introduced by Alan Albrecht of IBM in 1979. In 1984, Albrecht refined the method. The first Function Point Guidelines were published in 1984. The International Function Point Users Group (IFPUG) is a US-based worldwide organization of Function Point Analysis metric software users. The International Function Point Users Group (IFPUG) is a non-profit, member-governed organization founded in 1986. IFPUG owns Function Point Analysis (FPA) as defined in ISO standard 20296:2009 which specifies the definitions, rules and steps for applying the IFPUG's functional size measurement (FSM) method. IFPUG maintains the Function Point Counting Practices Manual (CPM). CPM 2.0 was released in 1987, and since then there have been several iterations. CPM Release 4.3 was in 2010.

The CPM Release 4.3.1 with incorporated ISO editorial revisions was in 2010. The ISO Standard (IFPUG FSM) - Functional Size Measurement that is a part of CPM 4.3.1 is a technique for measuring software in terms of the functionality it delivers. The CPM is an internationally approved standard under ISO/IEC 14143-1 Information Technology – Software Measurement.

Elementary Process (EP)

Elementary Process is the smallest unit of functional user requirement that −

  • Is meaningful to the user.
  • Constitutes a complete transaction.
  • Is self-contained and leaves the business of the application being counted in a consistent state.


There are two types of functions −

  • Data Functions
  • Transaction Functions

Data Functions

There are two types of data functions −

  • Internal Logical Files
  • External Interface Files

Data Functions are made up of internal and external resources that affect the system.

Internal Logical Files

Internal Logical File (ILF) is a user identifiable group of logically related data or control information that resides entirely within the application boundary. The primary intent of an ILF is to hold data maintained through one or more elementary processes of the application being counted. An ILF has the inherent meaning that it is internally maintained, it has some logical structure and it is stored in a file. (Refer Figure 1)

External Interface Files

External Interface File (EIF) is a user identifiable group of logically related data or control information that is used by the application for reference purposes only. The data resides entirely outside the application boundary and is maintained in an ILF by another application. An EIF has the inherent meaning that it is externally maintained, an interface has to be developed to get the data from the file. (Refer Figure 1)


Transaction Functions

There are three types of transaction functions.

  • External Inputs
  • External Outputs
  • External Inquiries

Transaction functions are made up of the processes that are exchanged between the user, the external applications and the application being measured.

External Inputs

External Input (EI) is a transaction function in which Data goes “into” the application from outside the boundary to inside. This data is coming external to the application.

  • Data may come from a data input screen or another application.
  • An EI is how an application gets information.
  • Data can be either control information or business information.
  • Data may be used to maintain one or more Internal Logical Files.
  • If the data is control information, it does not have to update an Internal Logical File. (Refer Figure 1)

External Outputs

External Output (EO) is a transaction function in which data comes “out” of the system. Additionally, an EO may update an ILF. The data creates reports or output files sent to other applications. (Refer Figure 1)

External Inquiries

External Inquiry (EQ) is a transaction function with both input and output components that result in data retrieval. (Refer Figure 1)

Definition of RETs, DETs, FTRs

Record Element Type

A Record Element Type (RET) is the largest user identifiable subgroup of elements within an ILF or an EIF. It is best to look at logical groupings of data to help identify them.

Data Element Type

Data Element Type (DET) is the data subgroup within an FTR. They are unique and user identifiable.

File Type Referenced

File Type Referenced (FTR) is the largest user identifiable subgroup within the EI, EO, or EQ that is referenced to.

The transaction functions EI, EO, EQ are measured by counting FTRs and DETs that they contain following counting rules. Likewise, data functions ILF and EIF are measured by counting DETs and RETs that they contain following counting rules. The measures of transaction functions and data functions are used in FP counting which results in the functional size or function points.