ISO/IEC 9126 Software engineering — Product quality was an international standard for the evaluation of software quality. It has been replaced by ISO/IEC 25010:2011. The fundamental objective of the ISO/IEC 9126 standard is to address some of the well known human biases that can adversely affect the delivery and perception of a software development project. These biases include changing priorities after the start of a project or not having any clear definitions of "success". By clarifying, then agreeing on the project priorities and subsequently converting abstract priorities (compliance) to measurable values (output data can be validated against schema X with zero intervention), ISO/IEC 9126 tries to develop a common understanding of the project's objectives and goals. The standard is divided into four parts: quality model external metrics internal metrics quality in use metrics. Quality The quality model presented in the first part of the standard, ISO/IEC 9126-1, classifies software quality in a structured set of characteristics and sub-characteristics as follows: Functionality - "A set of attributes that bear on the existence of a set of functions and their specified properties. The functions are those that satisfy stated or implied needs." Suitability Accuracy Interoperability Security Functionality compliance Reliability - "A set of attributes that bear on the capability of software to maintain its level of performance under stated conditions for a stated period of time." Maturity Fault tolerance Recoverability Reliability compliance Usability - "A set of attributes that bear on the effort needed for use, and on the individual assessment of such use, by a stated or implied set of users." Understandability Learnability Operability Attractiveness Usability compliance Efficiency - "A set of attributes that bear on the relationship between the level of performance of the software and the amount of resources used, under stated conditions." Time behaviour Resource utilization Efficiency compliance Maintainability - "A set of attributes that bear on the effort needed to make specified modifications." Analyzability Changeability Stability Testability Maintainability compliance Portability - "A set of attributes that bear on the ability of software to be transferred from one environment to another." Adaptability Installability Co-existence Replaceability Portability compliance Each quality sub-characteristic (e.g. adaptability) is further divided into attributes. An attribute is an entity which can be verified or measured in the software product. Attributes are not defined in the standard, as they vary between different software products. Software product is defined in a broad sense: it encompasses executables, source