More than just a picture: Icon interpretation in context
Siné McDougall
University of Wales Swansea
Swansea SA2 8PP
s.mcdougall@swansea.ac.uk
Martin Curry
National Air Traffic Control Services,
Bournemouth International Airport,
Christchurch BH23 6DF
martin.curry@nats.co.uk
ABSTRACT
Icon interpretation does not occur in isolation but in a wider, much more complex, context. In order to predict how
effective icons are likely to be in operational contexts we therefore need to extend our view of icon usability beyond the
confines of individual icon characteristics. The context of interpretation should encapsulate a range of factors including
task complexity and workload, the nature of the display as a whole, time of day, and user skills and preferences. This
paper reviews research which has examined contextual factors and formulates guidelines for design and a framework
for future research on the basis of this review.
Author Keywords
Icons, workload, preferences, user skills, perception.
INTRODUCTION
Guidelines and standards now provide icon designers with a wide range of recommendations about how icons can be
created to ensure that they are effective and usable (e.g. Horton, 1994). However, they focus primarily on individual
icons and rarely deal with the complexities of the wider context in which interpretation is likely to take place. The aim
of this paper is to examine the broader context of interpretation from a cognitive psychological perspective. The
framework for this discussion is shown in Figure 1. The role of perception and cognitive characteristics in
interpretation is considered but the primary focus is upon the role that context has in influencing perceived meaning.
The contextual factors considered include the effects of workload and task complexity, user preferences and aesthetics,
time of day effects, and user skills. In Figure 1 arrows represent links that have been established by research. Bold
links represent connections between general topic areas (e.g. cognitive characteristics and time of day) while dotted
links represent more specific connections (e.g. user preferences with icon concreteness).
PERCEPTION
Although a great deal of attention has been given to the way in which the size, shape and colour of icons can aid the
perception of icons, it is now recognised that the relationship between perception and interpretation is much closer than
previously thought. Recent research suggests that particularly meaningful icons in displays can capture our attention
and drive visual search (Mack, Pappas, Silverman and Gay, 2002). Because meaning can be extracted at the same time
as visual search, usability can be considerably enhanced by ensuring that key icons for interpreting displays ‘pop out’
from displays during search.
Workload
Sensory processing
Processing stages
Representational Codes
Response demands
Cognitive
Characteristics
Concreteness
Semantic Distance
Familiarity
Time of day
Effort
User Skills
Preferences
Marketing issues
Preference formation?
Perception
Search
Clustering
Semantic contrasts
Visual Conventions
Icon Interpretation
1
World Knowledge
Experience
Spatial ability
Visual attention
Pattern recognition
Visual learning & memory
Verbal learning & memory
Figure 1. Factors influencing icon interpretation.
Organising displays by clustering icon sets into meaningful groupings also helps users access meaning. This is readily
apparent to anyone using computer packages where icons with related functions are clustered in the same location.
Recent research has shown that icon clustering not only allows us to search displays more effectively (Niemela &
Saarinen, 2000), but also allows us to learn icon-function relationships more quickly (Richards & McDougall, 1999).
This is because clustering allows users to make inferences about categories of icon-function relationships and this, in
turn, aids the interpretation of individual icons within the clusters. Clustering therefore appears to be a very useful
design tool which is capable of delivering a number of benefits including workload reduction and enhanced visual
search and comprehension.
The relationship between perception and interpretation is also apparent when visual conventions are used to convey
meaning. For example, colour is commonly used to communicate meaning such as red for danger and green for go and
these colour conventions appear to have reasonable cross-cultural transferability (c.f. Courtney, 1986). Shapes are also
used to convey conventional meanings as a new visual language using icons is created: two arrows pointing left are now
commonly understood as ‘fast forward’; ‘contrast’ is indicated by a circle which is half white and half black; and hazard
is indicated by a combination of shape (triangle) and colour (red) conventions.
To summarise, user performance can be enhanced by ensuring that display design allows users to access meaning at the
same time as carrying out visual search. It is clear that there is considerable scope for expanding the ways in which
meaning can be conveyed implicitly through visual search. The use of visual conventions is also used to convey
meaning via perception. For these reasons, the interplay between interpretation and perception is likely to become
increasingly important in future display design.
THE ROLE OF COGNITIVE ICON CHARACTERISTICS
This section evaluates the role of cognitive icon characteristics in icon interpretation including icon concreteness,
semantic distance (the closeness of the icon-function relationship), and familiarity (see Figure 1). Research in the area
has tended to examine the effects of each characteristic in isolation. In order to arrive at a better understanding of the
role of cognitive icon characteristics in practice, the relative importance of these characteristics is evaluated along with
how they behave under high workload conditions.
Concreteness
The icon characteristic which has received most attention to date is icon concreteness, (i.e. the degree of pictorial
resemblance that an icon bears to its real world counterpart). The consensus is that concrete icons are easy to interpret
because they allow users to apply their everyday knowledge about the objects depicted in order to make inferences
about icon functions (Moyes & Jordan, 1993). Abstract icons, which have less obvious connections with the real world
(since they consist mainly of shapes, arrows and lines), are therefore more difficult to interpret. Using this logic,
concrete icons should be the best to use on graphical user interfaces because they seem more ‘natural’ to use and require
less explicit learning because of what we already know about everyday objects. However, studies which have sought to
find out more about the role of icon concreteness can be difficult to interpret. This is because, until recently, no
measures of concreteness were available and researchers had to rely on their own intuitions in order to decide whether
an icon was concrete or not. When creating concrete icon sets researchers tended to add more detail to pictorial icons
which meant they were also more visually complex (e.g. Arend et al, 1987; Stammers & Hoffman, 1991). This made it
difficult to decide whether concrete icons were easier to use because they were more pictorial or because the greater
detail in the icon made them easier to understand.
The roles which icon concreteness and visual complexity had in determining usability were resolved in later research.
Icon complexity determines how quickly users can search displays for appropriate icons. Because simple icons can be
distinguished on the basis of a few visual features, graphical interfaces containing simple icons are searched much more
quickly than those containing complex icons (Byrne, 1993). The use of concrete icons in displays, however, is
important in determining how accurately novices interpret icons, although differences in accuracy disappear as users
gain experience (McDougall, Curry & de Bruijn, 2000). As can be seen from Figure 2, ease of interpretation does not
depend on visual complexity or the level of detail in the icon.
Library
Lock
(a) concrete & complex icon
(b) concrete & simple
2
icon
Rinse
Eject
(c) abstract & complex icon
(d) abstract & simple
icon
Figure 2. Simple and complex, concrete and abstract icons.
Although the use of concrete icons appears to be useful in conveying meaning accurately, it is worth remembering that
a great deal of information cannot be represented in this way (e.g. 'eject', Figure 2d). Several studies have reported
difficulties in creating pictorial icons that are appropriate for abstract concepts (Howell & Fuchs, 1968; Jones, 1983;
Rogers & Oborne, 1987; Stammers, 1988, 1990; Stammers & Hoffman, 1991). In order to design abstract icons well
we therefore need to find other ways of representing information effectively without necessarily relying on visual
metaphors. One possible solution is to consider another characteristic, semantic distance, when selecting icons rather
than the degree to which they are pictorial.
Semantic Distance
Semantic distance refers to the closeness between the icon and the function it is intended to represent. Pierce (1932)
classified signs into 3 categories which can be summarised as follows:(i) Direct: where the icon-function relationship is close and visually obvious (see Figure 3)
(ii) Inferential: where inferences need to be made before the icon can be interpreted
(iii) Arbitrary: where the relationship between an icon and its function is entirely arbitrary.
A number of variants of this original tripartite classification of semantic distance have been proposed but it is probably
better to think of semantic distance of as a continuum which encompasses the complete range of icon-function
relationships from very close to very distant rather than as three distinct categories (McDougall, de Bruijn & Curry,
1999).
Heliport
(i) Direct
Entrance
(ii) Inferential
Biohazard
(iii) Arbitrary
Figure 3. Variations in semantic distance between icon and function.
Recent research suggests that semantic distance may be at least, if not more, important than concreteness in determining
icon effectiveness and ease of interpretation (Goonetilleke, Shih, On & Fritsch, 2001; Isherwood, 2002; McDougall,
Curry & de Bruijn, 2001). When Isherwood (2002) compared the relative importance of concreteness and semantic
distance, she found that semantic distance was the best predictor of novice user performance and accounted for over
50% of the performance variance observed. Novices appeared to rely primarily on the 'goodness-of-fit' between icons
and their functions, rather than their pictorialness, when learning icon-function relationships. One of the reasons why
semantic distance is more important than concreteness is because semantic distance accounts for a wider range of icons
which are not necessarily pictorial and may be quite abstract (as in Figure 3(ii))
Familiarity and User Experience
Another important, yet neglected, icon characteristic is familiarity. For icons, familiarity can be understood in two
ways:(i) familiarity with the objects depicted in the icon
(ii) the level of experience that an individual has with an icon.
The effects of user experience are rarely taken into account in the design process which concentrates primarily on the
creation of new interfaces which, by their very nature, have not been extensively used. Although users respond much
more accurately to concrete icons initially, differences in accuracy between concrete and abstract icons quickly
disappear as users reach skilled user performance with both types of icon (Green & Barnard, 1990; McDougall et al,
2000). This suggests that once users have gained a little experience, the use of visual metaphors in concrete icons has
little impact on accuracy.
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The effects of semantic distance may be more long lasting than those of concreteness (Goonetilleke, Shih, On & Fritsch,
2001; McDougall et al, 2001). Performance differences between icons with close and distant icon-function
relationships appear to remain for some time and it takes much longer for those using arbitrary icons to reach skilled
user performance. Interface designers therefore need to be aware of the 'lifecycle' of different icon properties and
consider the expected frequency of use in icon design.
To summarise, it is clear that cognitive icon characteristics play a significant role in determining the ease with which
icons are interpreted, particularly when users have little experience. Although the objects depicted in the icon are
important in accessing visual metaphors, the closeness of the icon-function relationship is fundamental in determining
icon efficacy.
INTERPRETATION IN CONTEXT
As we have already noted, to be able to inform design practice we need to understand how icon properties behave in
operational contexts and how they are affected by other factors such as workload, user preferences and skills, and timeof-day effects. The effects of these factors will now be considered.
Workload & Task Complexity
The vast majority of studies examining icon interpretation involve simple tasks which require little more than iconfunction matching. The use of operational equipment, however, often involves carrying out activities that are far more
complex and demanding. Obvious examples include the control of 'glass cockpit' aircraft (Mouloua, Deaton & Hitt,
2001), the operation of air traffic control systems (Remington, Johnston, Ruthruff, Gold & Romera, 2000), the
interpretation of military tactical displays (Kirkpatrick, Dutra, Lyons, Osga & Pucci, 1992) and dealing with process
control systems which regulate chemical and nuclear plants (Flach & Vincente, 1989). Given that the operational use of
such systems involves periodic conditions of high workload, it is surprising that so little research has examined the
effect of high workload levels on icon interpretation. The few studies that have been done suggest that interpretation is
degraded when workload is high although these deficits are considerably ameliorated by the skill and experience of
system users.
A recent series of experiments carried out by Isherwood, McDougall & Curry (submitted) examined how the factors
that affect icon interpretation change as workload is increased. These studies involved asking participants to carry out
an icon search-and-match task either on its own or while doing 1 or 2 other tasks simultaneously. Initially the more
tasks the user had to perform simultaneously, the poorer their performance. After gaining experience, however, similar
levels of accuracy were achieved irrespective of how many tasks participants had to carry out, but differences in
response times remained. These findings suggest that although the amount of effort required to select appropriate icons
reduces with experience, the extra processing time required to coordinate simultaneous tasks remains relatively
constant. Differences in response time probably remain because executive processing, or scheduling, is still required to
coordinate activity and deal with workload peaks even after users are experienced (see Gopher, 1996, for further
discussion).
Isherwood et al. also found that the cognitive characteristics which determined performance varied in accordance with
workload levels. For novice users semantic distance was most important under low workload conditions, because it acts
as an index of the ease with which icon-function relationships can be learned. Under high workload conditions,
performance was determined by object familiarity (i.e. prior experience of objects in the icon). This was because, when
workload is excessive, users are not able to learn icon-function relationships easily and so rely on their familiarity with
the object depicted in the icon. Generally, the impact of cognitive icon characteristics are less under high workload
conditions probably because it is task coordination, rather than the nature of the icons, that assumes more importance.
Although Isherwood et al's research tells us something about the likely impact of higher workload levels on icon
interpretation, both of the extra tasks used were visually based. Given that most complex tasks are likely to involve
different types of processing (i.e.. auditory or semantic processing in addition to visual processing), it is important to
consider what effects these processes might have on icon interpretation. One way to investigate this would be to draw
up a conflict matrix which pinpoints competition for resources in terms of sensory modalities, processing stages,
representational codes, and the response demands inherent in multiple tasks (see Wickens, 2002, for an example and
rationale). Workload demands could be systematically varied by increasing or decreasing the amount of conflict for
each of these resources. This would provide a more comprehensive picture of the workload effects on icon
interpretation and help the design of interfaces for complex tasks.
Time of Day Effects
The effects of time of day on a wide range of cognitive tasks has been well documented (Carrier & Monk, 2000). These
include visual search, memory and semantic processing (Folkard & Hill, 2002; Smith, 1992). Typically performance
improves over the morning then dips in the early afternoon. This source of performance variation therefore needs to be
taken into account in the design process not least because recent research suggests that users’ ability to interpret icons
may vary across the day.
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A number of studies have pointed out the link between the post-lunch dip in performance and the increase in car
accidents in the early afternoon (e.g. Lenne, Triggs & Redman, 1997). One thing which may underpin these
performance dips is the fact that drivers are slower to interpret and respond to road signs. Indeed, McFadden & Tepas
(1997) found diurnal variation in users’ ability to interpret road signs and their findings could not be explained by
factors typically used to explain road accidents such as the effects of fatigue, alcohol, or changing light levels. Other
research suggests that time of day effects may be most marked for icons which require greater processing effort (e.g.
visually complex and abstract icons; McDougall, Tyrer & Folkard, 2004). Designers may therefore need to create icons
sets that require relatively little processing where susceptibility to time of day effects have safety-related consequences.
Further research is needed to understand precisely how time of day affects icon interpretation and what the implications
are for safety-critical interfaces. Given the very real practical implications for system operation, it is surprising that so
little research has been conducted to examine whether time-of-day effects exist in icon interpretation.
User Preferences
As can be seen from Figure 1, user preferences are a further factor which has an impact on icon interpretation. There
are, sometimes, good reasons for choosing icons which users prefer when designing new interfaces.
Jordan (2000) has emphasised the importance of making interfaces and icons attractive to the user and a number of
ways have been devised to assess user preferences, particularly when a number of alternative icon designs are being
considered. Preferences provide a useful marker of how easy users think it will be to learn and use a new system.
Indeed users’ initial difficulties in interpreting icons may be reduced because an attractive interface gives the immediate
impression that the system is easy to use and increases the likelihood that the interface will be used on a regular basis
(Weidenbeck, 1999). The link shown in Figure 1 between preference and workload exists as much because users’
perceptions of workload are likely to be reduced by icon-based interfaces rather than any real reduction in workload. It
is important to note that strong preferences do not necessarily result in better performance. For example, users tend to
prefer concrete icon sets, even though performance with abstract sets is often just as good, if not better (Whiteside et al,
1985).
The role which users preferences are likely to play in design decisions is likely to vary with context of use. Preferences
will clearly be more important where the use of an interface depends primarily on its appeal (e.g. websites), while
interpretability will be more significant in safety- and time-critical interfaces (e.g. air traffic control displays).
It is worth noting that the aesthetic appeal of icons is often critical in determining which icons are chosen for use in new
packages. Icons are seen as a way of launching new products and as a vehicle for adding visual ‘glamour’ to what
might otherwise be a bland and uninteresting interface. Marketing concerns may therefore override principles of good
icon design in order to improve the marketability of products. If preferences and appeal are a primary factor in product
decisions we need to know more about how preferences are formed (see Figure 1) so that both aesthetics and
interpretability can be optimised.
User Skills
This section considers the skills individuals require to interpret icons effectively. Visual and spatial skills are thought to
be key skills needed to deal with graphical user interfaces (Subrahmanyam, Greenfield, Kraut & Gross, 2001). Spatial
skills correlate with performance in video game playing (Okagaki & Frensch, 1994) and the ability to divide attention
visually is also important (Greenfield, de Winstanely, Kilpatrick & Kaye, 1996).
It is an obvious over-simplification, however, to suggest that purely visuo-spatial skills are required for icon
interpretation. Verbal skills are also likely to be important although they have not really been considered.
Interpretation of letters and words, which are essentially specialised symbols or icons, require a wide range of verbal
skills including phonological (sound-based) skills, good verbal working memory and long-term memory skills (Smith,
Simmons & Kameenui, 1998; Swanson, 1999). On this basis, it seems sensible to suggest that icon interpretation will
involve a combination of both visual and verbal skills.
The visual skills required specifically for icon interpretation have not yet been investigated although it seems likely that
good visual memory skills would allow more effective learning of new icon sets, particularly if coupled with the ability
to use visual cues to understand the structure of the interface. Good verbal memory is also likely to be part of the skills
profile users’ require since icon interpretation is fundamentally about forming links between visual representations and
verbal concepts. Some measures of Verbal IQ may correlate well with users’ ability to learn icon-function
relationships. Subscales on the Wechsler Adult Intelligence Scale (Wechsler, 1997) such as the Vocabulary sub-test
(measures acquisition of verbal concepts) and the Similarities sub-test (measures ability to draw out conceptual
similarities between concepts) are likely to be good predictors users’ learning of icon-function relationships. Other,
more complex, factors such as the precise balance of visual and verbal skills required, the relative roles of working- and
long-term memory, and the relative importance of task-specific skills also need to be explored.
5
To date our understanding of the skills needed to work effectively with graphical user interfaces is limited and has
focussed on visuo-spatial skills. It is clear that a broader approach, which involves considering visual, verbal and
inferential skills, is needed in order to understand the relationship between user skills and icon interpretation properly.
GUIDELINES FOR DESIGN
Although we sought to deal solely with icon interpretation it is becoming increasingly apparent that perception and
interpretation overlap. Many interfaces already use grouping and location as ways of clustering icons meaningfully
together and research suggests that clustering icons, not only helps users to locate appropriate functionality, but also to
learn icon-function relationships more quickly and effectively. At a perceptual level, it is also important to be aware of
the increasing use of colour and shape conventions to convey meaning. There is every indication that these visual
conventions will be used more extensively as the 'language' of icons develops and expands. Designers therefore need to
have a good working awareness of what these conventions are, although it is worth bearing in mind that conventional
design does not necessarily mean good design.
On the basis of our review, it seems clear that the design process should pay particular attention to the effects of
cognitive icon characteristics on user performance. Considerable effort has gone into creating interfaces which employ
visual metaphors based on real-world experience. Although this approach has undoubtedly enhanced the usability of
many interface designs, difficulties arise when functions cannot easily be depicted pictorially. When icons must, of
necessity, be abstract designers need to ensure there is a good fit between the icon and its function (i.e. to reduce the
semantic distance between icon and function). Research has shown that the quality of the icon-function relationship has
a far greater effect on user performance than its pictorial content.
The exact cognitive characteristics that determine icon usability change as user experience grows. When interfaces are
used regularly, icon semantic distance and familiarity are the key design characteristics which must be considered. If
accuracy is critical, then semantic distance that will assume design priority. If, however, the system being used requires
time-critical responding, then icon familiarity requires primary consideration. In situations where icons are rarely
encountered, and icon functionality needs to be perceived instantaneously (such as for public information signs), then
the use of concrete icons are likely to be most appropriate. Contrary to popular opinion, concrete icons are unlikely to
provide any functional benefits for experienced users, even though they may be preferred.
It is also important to consider the demands that tasks are likely to make on users’ attentional resources. One useful
approach to take would be to use a conflict matrix to examine potential cognitive resource conflicts relating to sensory
modalities, processing stages, or representational codes.
Although the evidence to date is very limited, it may be that users of complex interfaces are particularly vulnerable to
errors and slow responding when tasks are carried out in the early afternoon. Time of day effects are a factor which
designers need to consider particularly if designing for complex interfaces that are likely to be in 24-hour use.
Almost all interfaces are now designed with a potential user group in mind. Understanding the skills needed for icon
comprehension is particularly important where the effort and investment required in order to train users is high (e.g.
pilots and air traffic controllers). Although aptitude tests for these types of user groups have been in place for some
time, research is still needed to explore what skills are required for dealing effectively with graphical user interfaces and
to identify 'skills gaps' which may make effective interaction difficult.
Another area where skills awareness is important is the development of interfaces for children. There is an expanding
literature which suggests that children can develop important cognitive skills as a result of using computers and there is
growing pressure that packages used by children should have educational payoffs. Knowing about the interrelationships between the skills needed for visual interface interpretation, and how to develop those skills through
design, is likely to become increasingly important.
Last but not least, it is important to consider user preferences and aesthetics in icon design. The icon with style, rather
than just functionality, is becoming increasingly important and is regarded as a key factor in launching new products.
Careful consideration of icon-function relationships, cognitive characteristics, user skills and task complexity may count
for little if the icon designs have no appeal.
A FRAMEWORK FOR FUTURE RESEARCH
Icon interpretation is inherently ambiguous because the relationship between icon and function is not determined by a
set of well-defined syntactic and phonological rules. It is also important to look beyond simple icon-function matching
to the wider context in which icon interpretation occur if we are to fully understand icon use in operational contexts.
The framework in Figure 1 was used to represent research progress to date and can also be used highlight areas were
further investigation is needed. Figure 1 suggests that the integration of understanding across areas of investigation is
sometimes limited. For example, potentially close links at a number of levels between perception and user skills have
not been investigated. Exploring these links is important if we wish to understand, and optimise, icon interpretation in
the complex contexts in which icons appear.
6
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