Perceptual structure cueing in a simple command language

Inf. J Man-Machine Studies (1984) 21, 19-29

Perceptual structure cueing in a simple command language S. J. PAYNE, M. E. S[ME AND T. R. G. GREEN

M R C / E S R C Social and Applied Psychology Unit, Department of Psychology, The University, Sheffield SI0 2TN, U.K. Computer languages are special cases of information displays. Successful information display, presented here as a mapping between the internal structure ol the information and its external representation, enables the reader to use perceptual features as cues to the internal structure. Such "perceptual parsing" is notably difficult or impossible in many command languages: instead, the structure of their inscrutable commands defies quick analysis. "Iaking a miniature context-oriented editor as a demonstration system, we show that an extremely simple and purely "surface" change to the syntax, namely putting operation codes in upper case to distinguish them from literais, produces a large decrease in error frequencies in each of the three experimental tasks. We conclude that designers should pay close attention to making the command structure easily perceived.

Introduction A MODEL FOR INFORMATION DISPLAY Psychologists who have studied the presentation of information have long advocated typographical cues as comprehension aids (Klare, Nichols & Shuford, 1957). Empirical evidence has shown that intelligent use of spatial layout and typeface can significantly improve comprehension and recall of instructional text (Hartley, 1978; Frase & Schwartz, 1979), tables (Wright, 1977), diagrammatic instructions (Szlichcinski, 1979) and sheet music (Sloboda, 1981). Typographical cues are not random: they are mappings between the internal structure of the text and its presentation in an external, written version. If the given class of material is regarded as a grammar generating "sentences" which have a "phrase structure", many of these various schemes for improved information presentation can be regarded as methods for mapping phrase structure relations onto spatial or typographical relations (Green & Payne, 1982). In general, information displays are easier to use when their structure is visible and can be "parsed" from purely perceptual cues--i.e, without having to determine the structural relations by reading the words and comprehending the semantics. This perceptual parsing appears to be an important activity which aids comprehension by allowing perceptual strategies (Bever, 1970) to be activated. Naturally, reading for different purposes demands that different aspects of the material be grasped, and so different mappings between structure and layout will be appropriate for different tasks, but any given task will be made easier by finding an appropriate mapping. The purpose of this article is to investigate two aspects of perceptual structure cueing in command language design. We do so by empirically comparing the usability of several versions of a miniature command language which exhibit different mixtures of 19 0020-7373/84/070019 + 11503.00/0

(~ 1984 AcademicPress Inc. (London)Limited

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the two aspects, looking for evidence that one or both aspects has an important effect on the usability of the language. LANGUAGE DESIGN AND BEHAVIOURAL EXPERIMENTATION

Perceptual difficulties abound in computer languages; some examples were given by Green (1980). One type of solution is the "pretty-printing" program, which can be used to reformat list structures or program text, mapping the structure onto spatial layout by indenting the deepest sublist or the innermost program components. Although these programs are widely used they have not been empirically examined. However, Norcio (1982) and Miara, Musselman, Navarro & Shneiderman (1983) have reported experiments on indentation of Fortran and Pascal. Both studies asked the perhaps rather simple question "Does indentation help?" rather than looking deeper; both studies found some benefit, in some conditions, from indentation, but it would have been more rewarding to discover whether the information highlighted by indenting was the most useful information [Fitter & Green (1979) question this] and whether indentation obscures any other useful information, such as relationships between the successive uses of variables which cut across indentation patterns. A thorough investigation of syntactic and spatial factors by Sheppard, Kruesi & Curtis (1981) also found that the mapping of program structure onto spatial layout affected comprehension, but this study once again was content with the demonstration of an effect rather than searching deeper. Other studies of program comprehension and information extraction in this area have typically varied more than purely perceptual factors [see the collection by Curtis (1981)]. Such studies have found difficulty in weighing the relative merits of various notations because of complex interactions between notation design and the particular comprehension task (Gilmore & Smith, 1984). Studies of command languages have investigated semantic aspects of names [see Black & Sebrechts (1981) for a review] but the purely perceptual features have been neglected, except for an experiment by Ledgard, Whiteside, Singer & Seymour (1980) whose interpretation is unclear: we shall mention this study in more detail below. Finally, Kahney (1983) analysed the program comprehension strategies of experts and novices by examining their recall of a briefly-seen short passage of program text. He argued that if expert programmers represent programs in higher-order chunks than novices [following the Chase & Simon (1973) account of chess expertise], and if the spatial layout gave strong cues to program structure, then experts' first recall attempts would contain material that could be deduced from layout, while novices' recall would simply list the first few words in order, as though they were recalling a poem. His results agreed with this analysis--although they also suggested that some novices quickly learn to use expert strategies. PERCEPTUALLY-DIFFICULT LANGUAGES

The evidence at present therefore indicates that perceptual structure cueing does, indeed, confer benefit; but the available evidence comes entirely from programs, extremely complex information structures. The aim of this article is to discover whether similar effects occur in extremely simple information structures. If so, then we shall have not only demonstrated the reliance of human information-processing on perceptual aids, but also have created a strong case for designing perceptual cues into computer languages at every level, from the simplest to the most complex.

PERCEPTUAL

STRUCTURE

CUEING

21

There are several examples of computer languages which are perceptually "diflicult"--i.e. lack any mapping between structure and spatial layout which could help the reader with perceptual parsing. For instance, the programming languages APL and Forth present famous difficulties to readers, as does the T,ECO text-editing language. Even conventional programming languages, such as Basic and Fortran, increase reader's difficulties by failing to allow a mapping between structure and layout. Ordinary flowcharts similarly obscure the structure (Green, 1982). All these languages, however, normally express rather complex tasks. Much simpler, and therefore much more suited to our purpose, is the typical context-oriented line-editor. Although context-oriented editors are not most people's favourite, they are widely used because they cost little in CPU resources and can be run over modems at low baud rates compared to screen editors (Meyrowitz & van Dam, 1982). Even if they were not widely used, they would make an excellent example of a simple structure in which perceptual cues are not usually available to readers. We shall introduce context-oriented editing with reference to the Unix ed editor, partly because it is now widely known and partly because this study was motivated in some degree by our amazement, when we first used ed, that such a simple system should give us so much difficulty. [We later found that we were not the first psychologists to be intrigued by ed's difficulties. Norman (1981) confesses to having used it for a year as an experimental material "to see how people deal with such confusing things".] The command

labcl searches through the text to find a line containing the string abc. This line becomes the "current" line. The command s/abc/def/ searches through the "current" line for the string abc and replaces it, if found, with the string def. Those two commands can be combined into /abc/s/abc/def/ which means "find the next line containing abc and in that line substitute abc with def". (Note the non-standard English syntax.) One does not have to try very hard to find potential difficulties with this type of command. Imagine that what you wanted was to search for the next line containing "s" and to replace that "s" by the string "s/s". The appropriate command is

Islslsls\/s/. The "\" is an "escape" symbol indicating that the following character, in this case " / " , is to be interpreted literally, not as a special symbol. As an exercise, try discovering whether the following command replaces "s\/s" by "s/\s" or vice versa:

/s\\\/s/s/s\\Vs/s\/\\s/. Besides being widely-used and perceptually difficult, there is another good reason for using context-oriented editing as an experimental task. A study by Ledgard et al. (1980) showed that the performance of both beginners and experts could be massively

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improved by changing the syntax. In their new syntax one of the above commands might have read FT "s" C "s" T "s/s" to be read as "Forward To next line containing 's'; Change 's' To 's/s' ". Ledgard et al. interpreted their results as showing that editors should be natural-language-like. A number of alternative interpretations were offered in correspondence following their publication. Although there is some evidence that certain information-gathering tasks are made easier if the operations can be internalized as short English sentences (Wright & Reid, 1973) we shall not discuss this interpretation here. Evidently at least one other view is possible since the Ledgard syntax gives much stronger perceptual cues to structure than the ed syntax. PERCEPTUAL DIFFICULTY: TWO HYPOTHESES

We offer two hypotheses, not mutually exclusive, to account for the difficulty of the " e d " editor. First, since the commands offer virtually no perceptual cues as to their grammatical phrase structure, the difficulty might be that perceptual parsing, as discussed above, is impossible. The unfortunate choice of backslash "\" and solidus " / " as special symbols exacerbates the situation: it is well-established that diagonal lines of opposite slope are more easily confused than a vertical line and a horizontal one (Olson, 1970); even when the backslash is not used, however, difficulties remain in discerning the structure of the command--i.e, in parsing it. Our second hypothesis introduces the notion of "overlap" between command symbols and literal symbols. For example the character "s" can represent the command "substitute" or the literal "s", its correct interpretation depending critically on its position within the command. Closely related to the notion of command-literal overlap is that of "character level ambiguity" (Thimbleby, 1982), in which a single character can represent more than one command depending on the mode of the editor. But the differences are as important as the similarities: command-literal overlap can occur in modeless command languages, unlike Thimbleby's character-level ambiguity, and although the ambiguity is still at the character level it occurs within, rather than between, transactions. Separating these two hypotheses empirically is far from straightforward. In this study we vary the degree of overlap within transactions, and vary the potential for overlap within experimental languages. The very exact and systematic variation of commandliteral overlap that we maintain enables us to evaluate our prime experimental variable, perceptual structure cueing, in situations where ambiguity is or is not a potential problem.

Experimental method EDITING TASKS

A simplified text-editing language was devised. Each command string operated on one lower case letter at a time, in a "document" containing a single line. Three editing tasks were used (see Fig. 1).

PERCEPTUAL

STRUCTURE

23

CUEING

Task 1 (command generation) Problem: the paper I~presents Solution: 3pd or Task 2 (command decoding) Problem: Problem:

3pD

(in the UC dialect) kryrhcb 2rBc (in the LC dialect) kryrhcb 2rbc

Solution: or

krycrhcb c kryJ.rhcb

Task 3 (inverse commands) Problem: Problem:

(UC dialect) 2dAs hdssdsw (LC dialect) 2das hdssdsw

Solution: 3sd

or

3sD

FIG. 1. Examples of each experimental task. Each problem sheet contained eight such problems; for each task there were six problem sheets with different degrees of command-literal overlap. Solutions in both dialects are shown for Tasks 1 and 3----subjects naturally only used the dialect they had learned.

Task 1, command generation: subjects had to write down a command to effect an alteration marked in a short phrase. Task 2, command decoding: given a seven letter string together with a command, subjects had to mark the effect of the command. Task 3, inverse commands: subjects were shown a command, and a string that had resulted from applying that command to an original unseen string. They had to generate a new command that would undo the effect of the given command and return the original string. In Task 1, only a single command was allowed; this restriction, and careful choice of materials, made the problem sufficiently well-defined. (Otherwise it would be possible to use "delete" and "insert" instead of "change", for example.) Similarly, in Task 3, ambiguous situations were avoided by careful choice of materials. For each task subjects completed six problem sheets, each containing eight examples. Between groups the sheets were completely identical except for the language conventions in Tasks 2 and 3. The order of presentation of sheets for each subject and task was balanced for learning effects. THE EDITING LANGUAGE A typical command was 3dBc This command means "find the 3rd 'd' and put a 'c' Before it". Two dialects were used, in one of which, the Upper-case dialect, the operation codes were always in

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ET AL.

upper case, as in this example. In the Lower-case dialect the command would read 3dbc The general structure of each command was N P O Q , where N was a single digit and P, O, Q were single letters. Five operations were possible, two of which, Delete and Swap, needed no O part: (insert) after (insert) before change delete swap Examples: lvau finds 3zbh finds 2wcy finds 3xd finds 2ms finds

a or A b or B c or C d or D s or S the the the the the

first v and inserts u after it third z and inserts h before it second w and changes it to y third x and deletes it second m and swaps it with the letter immediately following

The instructions to subjects included a full description of the language, stressing its consistent constituent structure. EXPERIMENTAL DESIGN It is impossible to vary either of the two experimental variables, perceptual structure cueing and command-literal overlap, independently of the other: changing the overlap will add a perceptual cue of some sort, and vice versa. We therefore used two groups (each of 10 subjects) with different levels of structure cueing between groups, and varied the degree of overlap within groups. The LC group used the Lower-case dialect of the editing language, in which operators and literals belong to overlapping sets and can be confused. The UC group used the Upper-case dialect with less potential for confusion between operators and literals, although some degree of confusability exists since letters are mentally coded by both perceptual and symbolic codes (Posner, Boies, Eichelman & Taylor, 1969). In both groups the degree of confusability was varied systematically across the six sets of problems. Sheets 1 and 2: neither literal belonged to the set of operator codes; e.g. 2mcn. Sheet 3: one literal belonged to the set of operator codes, but it was not the operator used in that command; e.g. 2acn. Sheet 4: one literal was the same as the operator, the other was not a member of the set of operators; e.g. 3fcc. Sheet 5: both literals were members of the set of operators, but neither was the actual operator of the command; e.g. 2acb. Sheet 6: both literals were members of the set of operators, and one was the same as the operator of the command; e.g. 2bcc. Overlap could therefore occur on problem sheets 3-6. To assist in controlling the frequencies of particular commands, two of the eight examples on these sheets had

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no overlap, exactly as in sheets 1 and 2. In every sheet, each of the five operators was used either once or twice. The position of the target letter for each example was chosen in such a way that the sum of the pointer digits for each sheet varied by no more than two. DEPENDENT VARIABLES Instructions to the subjects stressed both speed and accuracy; time and errors were used as dependent variables. Times were recorded by the subjects themselves, writing the starting time and finishing time for each problem sheet, taken from a large digital counter which advanced one unit every 4 s. Errors were scored by the experimenter from the response sheets. SUBJECTS

Twenty young people aged from 19 to 26 years, all undergraduates or recent graduates of Sheffield University, participated in the study. Subjects were allocated to experimental groups in their order of arrival.

Results ERRORS

Error scores are displayed in Table 1. By inspection, differences in command-literal overlap have no observable effect on performance, while the difference between the LC and U C dialects has a substantial effect. TABLE 1

Total errors for each task and overlap condition Overlap condition (sheet number) 1

2

3

4

5

6

Task 1

LC UC

9 0

7 4

9 4

16 5

10 2

9 4

Task 2

LC UC

3 0

1 0

1 0

3 0

3 0

3 0

Task 3

LC UC

10 3

7 1

8 0

9 2

10 1

6 3

To test for differences between editing dialects, the total errors made by each subject in each group were compared, task by task. Examining the individual data for statistical analysis, the error scores were as usual steeply positively skewed with J-shaped distributions. In such cases, the mean, variance and skew are closely related: a group with a high mean usually also has a high variance and a large skew. The hypothesis of interest is that one of the treatments will increase the likelihood of errors, which in these circumstances will mean that the mean of the distribution will increase and that the variance will increase similarly. To compare treatment effects of the two

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dialects, a parametric comparison of variances will serve for Tasks 1 and 3 - - b u t in Task 2 the UC group made no errors at all, giving a zero variance and an infinite variance ratio. The comparison was therefore bolstered with a distribution-free comparison of central tendency, an inefficient but acceptable statistic for these purposes. Results were as follows. For the variance ratio test, the LC group's variance was significantly greater on Task 1 ( F = 14.53, d.f. = 9 , 9, P = 0 . 0 0 0 5 ) and on Task 3 ( F = 8 . 1 8 , d.f.=9, 9, P = 0 . 0 0 4 4 ) ; the test was inapplicable on Task 2, as we have noted. Comparing central tendency using the Mann-Whitney rank-sum test, the groups did not differ significantly on Task 1 but the LC group made significantly more errors on Task 2 ( P < 0 . 0 1 ) and Task 3 ( P < 0 . 0 5 ) . These figures can be interpreted as evidence that the likelihood of errors was significantly greater for the LC group on every task. T o test for differences between overlap conditions, Friedman two-way rank-sum tests were performed on the data of Table 1 for each combination of task and group. No significant effects of overlap were found. TIMES

Figure 2 displays the average time taken for each overlap condition in each of the three tasks for both groups. Although subjects in the Upper-case group performed faster on average, analysis of variance revealed no significant main effects or interactions, either on transformed data or raw scores. However, the direction of the visible difference clearly indicates that the demonstrated effect for errors was due to a genuine performance difference and not to a tactical trade-off between speed and accuracy. The UC group not only performed more accurately, but also performed at least as fast as the LC group. 1,50

II0

I--

~

LC UC

uc

70

uc Tosk I 30

I I

t 2

I 3

I 4

Tosk 2 t 5

I 6

I I

I 2

I 3

I 4

Tosk 3 I 5

I 6

I I

I 2

I 3

I 4

I 5

t 6

FIG. 2. Times to complete one sheet of eight problems, for the U C and LC dialects. Overlap conditions 1-6 are defined in the text.

Discussion In view of the simplicity and low statistical power of our experiment, the results should be conservatively interpreted. We found that:

PERCEPTUAL STRUCTURE CUEING

27

1. changing the degree of overlap between operator symbols and literals within command strings made no perceptible difference to times or errors; 2. perceptually cueing the structure of the command string, by using an upper-case command (which eliminates potential overlap between operations and literals) reduced both times and errors. Without making any further attempts to interpret the results, the bald fact that a minute surface change can have a dramatic effect on usability should be treated with urgency by designers of computer languages. It suggests that issues such as delimiter design should receive far more attention, in the interests of achieving a clearly visible structure for each command. PERCEPTUAL COMPLEXITY AND PROBLEM COMPLEXITY The results of this experiment illustrate once again that language designers should not think in terms such as "syntactic sugar", implying that "deep" problem-solving complexity can be readily separated from "surface" perceptual complexity. Instead, a change that affects only the surface information (choice of upper or lower case command letter) has been found to affect problem solving success at a much deeper level than one would expect. Similar findings have, of course, been reported before: for instance, Brooke & Duncan (1981) showed that fault-finding in digital circuits was improved greatly on all measures by a simple surface change in the notational conventions, allowing connectors to travel diagonally instead of only vertically and horizontally; Fitter & Richards [reported by Green et al. (1981)] made an even simpler change in a notation, asking subjects to construct a route from cards saying "A goes to B--B goes to C", etc., or "reach B from A--reach C from B, etc.", and again found a surprisingly large effect on performance. Perhaps the difficulty lies in extracting information from a perceptually confusing notation, i.e. in reading the written material; or perhaps it lies in mental operations and manipulating visual images while solving problems. In either case, it ought not to be surprising that perceptual complexity affects overall problem-solving speed and accuracy. Nevertheless it is as well to be frequently reminded that human information processing, unlike the average computer program, operates not on a purely formal stream of symbols, free from connotation and free from context, but on shape, form and remembrance of things past. TRANSACTION DIFFICULTY AND USABILITY The experiment fails to demonstrate any difference in difficulty between commands where overlap occurs and those where overlap could, in principle, occur, but does not in practice. However, when a change to the language dictates that overlap can n e v e r occur the performance improvement is truly dramatic. It is, of course, very difficult to uncover whether this is because perceptual parsing is easier, or because there is less potential for confusion between operators and literals. Very probably both effects take place. Whatever the relative salience of the two effects, our results--large differences between groups but no differences between overlap conditions--lead us to suggest that the overall usability of a particular command is influenced by the difficulty of

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using it for certain transactions. One could readily explain such a finding. Because some transactions are more diffieult, they force the user to adopt more powerful but more demanding processing strategies. These strategies have to be used throughout, so that the difficulty of a particular transaction leaks out and colours other transactions using the same command. Our experiment does not address this contention very strongly, but we do think the suggestion deserves a slight digression. If the usability of a command depends on its suitability for particular (difficult) transactions, might we expect that the usability of a language will be determined by the ease of using particular commands? The situation is certainly more complex in this case; users can and often do avoid certain commands altogether, adopting alternative strategies which utilize only those commands they know and love. However, this avoidance strategy is not always possible, and is probably never possible without some cost to the overall usability of the system. Through a complex medium of attitude, motivation, processing strategy, etc., the usability of one particular part of a command system may have a considerable effect on the usability of other parts. The study of such interactions may assume an important role in future HCI research.

Conclusions The model of information display in terms of perceptual structure cueing, with which we began this article, has been shown to extend to a miniature command language. Two hypotheses were advanced, one that perceptual difficulty of a language was caused by lack of cues to permit perceptual parsing; the other, that the difficulty was caused by command-literal overlap. The experimental results showed a large effect from adding cues to permit perceptual parsing, and no perceptible effect at all of command-literal overlap. Thus, a small syntactic change can produce a very large increase in usability, just by revealing the structure of the command more clearly. Designers should pay close attention to this point. The result also bears on the psychology of problem-solving, adding to the weight of evidence that perceptual difficulties may take a surprisingly high toll.

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