Usability consultant Jakob Nielsen and computer science professor Ben Shneiderman have written (separately) about a framework of system acceptability, where usability is a part of "usefulness" and is composed of: Usability is often associated with the functionalities of the product (cf.ISO definition, below), in addition to being solely a characteristic of the user interface (cf.framework of system acceptability, also below, which separates usefulness into usability and utility).For example, in the context of mainstream consumer products, an automobile lacking a reverse gear could be considered unusable according to the former view, and lacking in utility according to the latter view.Usability includes methods of measuring usability, such as needs analysis and the study of the principles behind an object's perceived efficiency or elegance.In human-computer interaction and computer science, usability studies the elegance and clarity with which the interaction with a computer program or a web site (web usability) is designed.In software engineering, usability is the degree to which a software can be used by specified consumers to achieve quantified objectives with effectiveness, efficiency, and satisfaction in a quantified context of use.The object of use can be a software application, website, book, tool, machine, process, vehicle, or anything a human interacts with.
This position is sometimes illustrated with the remark that "The only intuitive interface is the nipple; everything else is learned." Instead, he advocates the term "intuitable," i.e., "that users could intuit the workings of an application by seeing it and using it." He continues, however, "But even that is a less than useful goal since only 25 percent of the population depends on intuition to perceive anything." ISO/TR 16902 ("Ergonomics of human-system interaction—Usability methods supporting human-centered design") is an International Standards Organization (ISO) standard that provides information on human-centered usability methods that can be used for design and evaluation.A usability study may be conducted as a primary job function by a usability analyst or as a secondary job function by designers, technical writers, marketing personnel, and others.