An experimental evaluation of prefix and postfix notation in command language syntax

Abstract

Commands in a command language usually consist of a verb plus object(s). A designer must decide the word order in which elements will appear in the command string. Many existing command languages use the format, verb-object, often referred to as prefix notation. However, another feasible syntax is object-verb, referred to as postfix notation. The objective of this study was to determine whether one of these notations facilitates user performance. Two command languages for a text editor were designed. One is based on the natural word order of English, verb-object. The other is based on the reverse word order, object-verb. Sixty subjects, all native speakers of English, were blocked into three groups: novices, experienced subjects who normally used a line editor, and experienced subjects who normally used a screen editor. Subjects were randomly assigned to use one of the command languages. They learned to use the command language by self instruction, then were given 20 min to perform a manuscript editing task. The task was deliberately made too long for any subject to finish. Measures of performance used were: percentage of task completed, percentage of erroneous commands, and editing efficiency. A two-way ANOVA was performed on each of the three dependent variables. There were no significant differences in performance between subjects who used the prefix command language and those who used the postfix command language, contrary to intuitive expectations. The novices differed significantly from the experienced subjects on percentage of task completed and percentage of erroneous commands, as expected. However, there was no significant difference between the novices and experienced subjects on the measure of editing efficiency. Variables which were correlated with percentage of task completed strongly enough to be useful as covariates were typing speed (all subjects) and contact hours with text editors (experienced subjects).