Function Point

A function point is a unit of measurement to express the amount of business functionality an information system provides to a user.
Function points are an ISO recognized software metric to size an information system based on the functionality that is perceived by the user of the information system, independent of the technology used to implement the information system.
Function Points were defined in 1979 (Function Points: A New Way of Looking at Tools) by Alan Albrecht at IBM.
The method of measuring the size of an information system and expressing it in a number of function points is called Function Point Analysis. The method is kept up to date by worldwide cooperating FPA user groups like NESMA and IFPUG.
Basic function points are categorized into five groups: outputs, inquiries, inputs, files, and Interfaces. A function point is defined as one end-user business function, such as a query for an input. This distinction is important because it tends to make a function point map easily into user-oriented requirements, but it also tends to hide internal functions, which also require resources to implement. To make up for this (and other) weaknesses, some refinements to and/or variations of the basic Albrecht definition have been devised, including
Early and easy function points. Adjusts for problem and data complexity with two questions that yield a somewhat subjective complexity measurement; simplifies measurement by eliminating the need to count data elements.
Engineering function points. Elements (variable names) and operators (e.g., arithmetic, equality/inequality, Boolean) are counted. This variation highlights computational function [Umholtz 94]. The intent is similar to that of the operator/operand-based Halstead measures (see Halstead Complexity Measures).
Bang measure. Defines a function metric based on twelve primitive (simple) counts that affect or show Bang, defined as "the measure of true function to be delivered as perceived by the user" [DeMarco 82]. Bang measure may be helpful in evaluating a software unit's value in terms of how much useful function it provides, although there is little evidence in the literature of such application. The use of Bang measure could apply when reengineering (either complete or piecewise) is being considered, as discussed in Maintenance of Operational Systems--An Overview.
Feature points. Adds changes to improve applicability to systems with significant internal processing (e.g., operating systems, communications systems). This allows accounting for functions not readily perceivable by the user, but essential for proper operation.