| Let us further examine the keywords used in the following meaning-packed definition of usability, as well as the essence of those words: "Usability is the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use." Specified users: No product can be designed to suit everybody. At the outset of design, it is critical that the key user groups be identified. A software system can be optimized for one or two user groups, but any attempt to design it for more groups is a guarantee of failure. Specified goals: The key goals of each user group must be documented. Note that the user goal is different from the business or technical goals. For example, a data-entry application's business goal is to eliminate paper records and reduce maintenance cost. A potential technical goal could be to use the latest technology. The users' goal is to be able to enter an acceptable number of records with as few errors as possible. Effectiveness: Effectiveness is a measure of the use of a system in relation to the accuracy and completeness achieved by meeting the users' goals and subgoals. For example, the measure of the task effectiveness of the data-entry operator is calculated as the percentage of two attributes: the number of records entered and the quality (mistakes, formatting, and so on) of records entered. Efficiency: Efficiency is the measure of effectiveness at the rate of resource utilization. This is a valuable economic indicator. Continuing our example, when time is unlimited all the records can be entered with 100 percent effectiveness, but resources are always a constraint. Regardless, records need to be entered in a finite amount of time. In our example, the efficiency is calculated as the ratio of the effectiveness and the time taken to enter the records. Satisfaction: Satisfaction includes comfort and acceptability of use. Comfort refers to users' overall psychological and emotional responses to system use (whether users feel good, warm, and pleased or tense and uncomfortable). Acceptability of use may measure either overall attitude toward the system or the user's perception of specific aspects, such as whether or not users feel the system supports the way they carry out their tasks, whether or not they feel in command of the system, and whether or not the system is helpful and easy to learn. If satisfaction is low while efficiency is high, it is likely that the users' goals do not match the goals selected for the measurement of efficiency. Specified context of use: Specified context of use is the technical, physical, social, and organizational environments in which users interact with the system.
| The ISO 9241 11 definition of usability: "Usability is the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use." |
Usability is the key quality objective of any system. Like other quality attributes of the system, it cannot be easily retrofitted as an afterthought. Usability is a systemic attribute and begins with project goal setting and system requirements gatherings. Clearly, if usability requirements are not adequately captured up front, the cost of introducing them, either by choice or by user demands, may be severe and at times prohibitive. The terms that have been used to define usability are many and include usability design engineering, user-centered design, human-centered design, usage-centered design, human computer interaction, user experience, and customer experience. Most of these terms strive to create their own niches, thereby hoping to increase their acceptance in organizational practice. Though it is beyond the scope of this book to provide a detailed view of the different practices, in this chapter we present key concepts from the different practices to create an overall view of usability practices. |
