Background

Syntax at the Communications Level is an important issue in the design of interactive dialogues between the computer and user. A number of previous studies have focussed upon syntax and have attempted to establish the conditions under which particular sequences are preferred. Clearly such an endeavour has a role in influencing system design of the future. A number of studies performed by Barnard and colleagues have focussed on the structure and content of interactive dialogues. Those concerned with structure are of particular interest here.

One study investigated the ordering of arguments within a command sequence (Barnard, Hammond, Morton and Long, 1981). The application that was investigated was a message decoding system in which each command had two arguments; one of these arguments would be the direct object and the other the indirect object of the command e.g.

ERASE DIGIT 9 MESSAGE 7

[command] [direct object] [indirect object]

This command would cause digit 9 from message 7 to be erased. Three factors concerning syntax were of interest:

i) the relationship between the argument order and the order used in natural English. ii) the consistent positioning of the recurrent argument.

iii) the relationship between argument order and the position of argument values on a VDU. In the interactive, on-line version of the task it was found that positionally consistent systems were the most readily learned, but only if the recurrent argument was found in the first position. An effect due to the position of the direct object (i.e. that it should be placed so as to be compatible with natural English syntax) was found for the positionally inconsistent systems, but then only for certain commands. On average, the positionally inconsistent group having an "unnatural" structure performed no worse than the group which had a "natural" structure. This study illustrates that the commonly held assumption that device syntax should adhere to the principles of natural language is not necessarily founded.

Another study by Barnard and Hammond (1982) investigated syntax, but here syntax between command and its argument. A menu-driven data-base application was studied. The system had five functions and five objects. The data-base could be interrogated by selecting a command sequence consisting of one of the functions and one of the objects. The positioning of

the items in the menus was balanced across subjects (either the function set or the object set appeared as the top half of the menu). Subjects received instructions which were balanced such that half mentioned the object first and half the function first. Prior to the

experimental session subjects were briefed about the system; half the subjects were told about the system's functions and half about the system's objects.

Two versions of the experiment were run. In the first version, the subjects were allowed to use 'free' syntax i.e. they could choose whether to specify the object or the function first. Here an overall bias was found towards a dialogue in which the function is selected before the object. This bias was modified by the type of briefing the subjects had received (i.e. subjects who had been told about the objects were more likely to select the object first). There was also a tendency for subjects to pick menu items from the top half of the display first and to choose the item mentioned first in the instructions as the first item in the command sequence. In the second version of the experiment the required syntax was fixed, half the subjects had to specify the object first and the other half were required to specify the function first. Under these conditions it was found that the initial briefing and the structure of the questions had less marked effects on performance. Also there was no performance difference between the two syntax groups. For the 'fixed' groups compatibility with natural language syntax does not seem to be important.

A comparison between the two groups, 'fixed-' and 'free-' syntax, revealed that there was a difference in the learning of the system for the two groups. Subjects in the 'free-syntax' group seemed to acquire less knowledge about the system: they requested 'Help' more often in the early stages of use and experienced more difficulty in transferring from simple to complex problems.

The differing results for the two versions of the experiment were explained in terms of the differential task demands imposed by the two variants of the system.

A further study concerning syntax was reported by Cherry (1986). Subjects were required to use one of two command languages designed for a text editor. The two languages differed in their syntax; one being object-action and the other action-object. (NOTE: due to the

application the objects were numbers which referred to the line num ber/w ord number etc. e.g. 'insert 25'; 'replace 5 ,7). Contrary to intuitive expectations, there were no differences in performance between the two syntax groups on any of the measures taken (percentage of tasks completed; percentage of erroneous commands; editing efficiency).

These latter two studies show that in one menu system and one command-line system there was no effect due to syntax (o-a vs. a-o). However, the preference for an action-object syntax

— r . — ~

in the 'free* syntax condition of the Barnard and Hammond study indicates that the two orderings are not always found to be equivalent and factors, such as cognitive load imposed by the task, might be responsible. Clearly the relative usability of the two syntax structures over two input styles cannot be assessed from these previous studies which are set in different application domains.

It has been suggested in a paper that describes a kit for generating interfaces (Lieberman, 1985) that a good principle for interface design is to provide the user with both options (object-action and action-object). The kit (EZWin) allows this type of interface to be developed. However, the consequences of this approach may not be favourable as the Barnard study showed a learning deficit in the free syntax condition. A trade-off between time to learn and ease-of-use once learned, might have to be considered.

Given the above studies on syntax, it would seem that there is little empirical evidence to support the notion that an object => action syntax is easier to use. However, it is also true that they did not demonstrate a clear advantage for action => object syntax. So the

preference for object => action syntax found in the observational study cannot be explained with reference to these previous studies. It is interesting to note that many new commercial systems have followed the lead of the Xerox Star and have incorporated an object => action syntax; deeming it to be more 'natural' to use. Clearly there is a need for further empirical investigation into the use of syntax to determine the conditions under which a particular sequence is advantageous.