The study of human attention may be divided into three components. These are alertness, selectivity, and processing capacity. This paper outlines experimental techniques designed to separate these components and examine their interrelations within comparable tasks. It is shown that a stimulus may be used to increase alertness for processing all external information, to improve selection of particular stimuli, or to do both simultaneously. Development of alertness and selectivity are separable, but they may go on together without interference. Moreover, encoding a stimulus may proceed without producing interference with other signals. Thus, the contact between an external stimulus and its representation in memory does not appear to require processing capacity. Limited capacity results are obtained when mental operations such as response selection or rehearsal must be performed on the encoded information.

If a stored letter can be matched more rapidly with a physically identical letter (e.g., AA) than it can with a letter having only the same name (e.g., Aa), then the stored representation must preserve something of the visual aspect of the letter. Immediately after the presentation of a letter, a physical match is about 90 msec. faster than a name match and this difference is lost after 2 sec. An interpolated information processing task abolished the difference between physical and name match RTs, but visual noise alone does not affect this difference. When the visual aspect of the letter is made a completely reliable cue, the efficiency of a physical match is maintained more adequately. If only the name of the 1st letter is presented, Ss show the ability to recode the information into a form which is as efficient as a physical match and more efficient than a name match. Consideration is given to the relevance of these findings to the general questions of decay, rehearsal, and generation of visual codes.

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This study is a comprehensive research effort aimed at evaluating a computer system design methodology. It reports on the 1984 Olympic Message System (OMS), a voice mail system that was developed according to three behavioral principles. It describes a project from start to finish-from design and development to actual use by the customer. This research is unique in that part of its purpose was to carry out a case study of system design methodology. Consequently, the research effort involved keeping a diary; recording observations, results, and personal feelings; retaining early versions of materials; and building a usage analysis recording system in the final product and carrying out the analyses later.Fifteen behavioral methodologies used to achieve good usability are described. The dates, times, methodologies used, and numbers of people involved, as well as results, explain how the system development actually proceeded. Ali aspects of usability evolved in parallel and under one focus of responsibility. We also mention some mistakes made and how the hehavioral methodologies allowed us to identify and recover from them. This makes it possible for the reader to learn when and how a particular methodology contrihutes to the design process, and how long i( may take to carry out. PRINCIPLES OF SYSTEM DESIGNIn the past decade, we have been trying to arrive at procedures that couid he used to develop computerbased systems that are reliable, responsive, easy to learn, useful, and desirable. We have recommended three principles [10,11] to test this research: