On the Psychological Importance of Time in a Time Sharing System1 2

One of the most important problems in the design and/or operation of a computer utility is to obtain dynamical characteristics that are acceptable and convenient to the on-line user. This paper is concerned with the problems of access to the computer utility, response time and its effect upon conversational use of the computer, and the effects of load on the system. Primary attention is placed upon response time; rather than a single measure, a set of response times should be measured in a given computer utility, in correspondence to the different types of operations requested. It is assumed that the psychological value of short response time stems from a subjective cost measure of the user's own time, largely influenced by the value of concurrent tasks being postponed. A measure of cost (to the individual and/or his organization) of the time-on-line required to perform a task might thus be derived. More subtle is the problem of the user's acceptability of given response times. This acceptability is a function of the service requested (e.g., length of computation), and variability with respect to expectations due both to uncertainty in the user's estimation and to variations in the response time originated by variable loads on the system. An effort should be made by computer-utility designers to include dynamic characteristics (such as prediction of loads and their effects) among their design specifications.