PREDICTING THE USABILITY OF ALPHANUMERIC DISPLAYS
TULLIS, THOMAS STUART
Doctor of Philosophy
A review of the literature on alphanumeric displays, especially computer-generated displays, suggests that four basic characteristics of display formats affect how well users can extract information from the displays: (1) Overall density--the number of characters displayed, expressed as a percentage of the total spaces available; (2) Local density--the number of other characters near each character; (3) Grouping--the extent to which characters on the display form well defined perceptual groups; and (4) Layout complexity--the extent to which the arrangement of items on the display follows a predictable visual scheme. Objective ways of measuring these display characteristics have been developed and implemented in a computer program. In Experiment 1, 520 computer-generated displays that varied on these display measures were studied. Search times to locate data items on the diplays were measured as well as subjective ratings of ease of use. Regression equations were developed to predict the search times and subjective ratings using the display measures. The results indicated that both search times (R('2) = .508) and subjective ratings (R('2) = .805) could be predicted quite well. In experiment 2, the regression equations developed in Experiment 1 were used to predict, a priori, search times and subjective ratings for a new set of 150 displays. The regression equations generalized quite well, resulting in high correlations between predicted and actual search times (r = .800) and subjective ratings (r = .799). The regression equations indicate that the most important predictors of search time are two measures associated with the grouping of characters: the number of groups on the display and the average visual angle subtended by those groups. Likewise, the most important predictors of subjective rating are a measure of local density, which is essentially how "tightly packed" the display is, and a measure of layout complexity, which is essentially how well the items on the display are aligned with each other.