ELSEVIER

PII: SOOO3-6870(97)00028-8 Applied Ergonomics Vol29, No. I, ~~75-82, 1998

C 1997 Elsevier Science Ltd Printed in G&Britain. All rights reserved

0003-6870198 $19.00 + 0.00

Usage centred research for everyday product design

H. Kanis School of Industrial Design Engineering, Department of Ergonomics, Delft University of Technology, Jaffalaan 9, 2628’ BX Delft, The Netherlands

(Received 31 July 1996; in revised form 19 January 1997)

Prospective users of a new design in the area of everyday products offer innumerable opportunities for measuremenit and observation, in view of both the diversity in user populations and the freedom of where and how to use a product. In this paper, the relevance of human data is assessed for their impact in meeting functional imperatives in a design. On the basis of empirical studies, the significance of the observation/registration of user activities, including perceptual and cognitive activities, and the use of actions actually carried out is demonstrated. For everyday products, these activities are found to be only loosely related to human characteristics such as sensory capacities, body dimensions and exertable forces. Such characteristics seem to combine a limited relevance for usage centred design with relatively easy measurability. In contrast, observation of user activities may be evasive and is often laborious. User trialling is seen as an obvious way to enable designers to accommodate prospective user activities in everyday product design. 0 1997 Elsevier Science Ltd

Keywords: design, everyday products, human characteristics, use actions, user trialling

Introduction

In discussing the usefulness of human factors studies for the design of systems, such as in work conditions, Chapanis wrote in 1988, re-emphasizing what he had observed in 1971: “All too often dependent variables are picked because they are easy to measure, because instruments or devices for measuring them are available, because the investigator or other people have used them, or simply because they are most likely to yield statistically significant results.” (p.261). Chapanis argues that the phenomena which are studied often “have little or nothing to do with the criteria that are important in the design world.” (p.262).

To what extent these worrying observations hold for usage centred design of everyday products is hard to demonstrate since this type of product rarely features in Ergonomics/Human Factors (E/HF) research papers. But there seems not much reason to feel confident that in this area E/HF research. would resist the lure of measurability which tends to dominate academic endeavours in empirical studies. In fact, (prospective) users of everyday products offer innumerable opportunities for measurement and observation, in view of both the diversity in user populations, and the freedom of where and how to use a product, rather than job selection and training in specific tasks as may be the case for occupational conditions. Thus, the study of a particular user-product interaction may be related to such diverse data as

(a) sensory capacities and anthropometrics, physio- logical functions, e.g. heart rate and oxygen con- sumption, and human performance like exertable forces and reaction times, as well as (b) the activities of humans when actually using a product, e.g. reasoning and physical actions, includ- ing the effort involved and the possible experiencing of inconvenience.

The (a)-type of data may be found in standard reference texts like handbooks, as human characteristics. The de- sign relevance of such data tends to be the subject of on- going debate (cf. Chapanis, 1988). As to everyday products, Green et al, 1997, pose that in design the use- fulness of human characteristics is largely limited to set- ting general boundary conditions, and that there is very little in handbooks which seems to be used by designers in coming to grips with the actual activities of users in operating a particular design, as then data of the type (b) are needed, i.e. in terms of user activities. This obser- vation implicitly questions the significance of some er- gonomics research for design, at least in the area of everyday products. It would seem that ergonomics re- search, in keeping up with scientific traditions, tends to produce data which are not very significant for de- signers who seek information which makes sense for a particular design at hand. This alleged distinction in re- levance of human data for everyday product design, i.e. between user activities and human characteristics, con- stitutes the topic of this paper.

16 Usuge cmtred research jar evrryduy product design: H. Kanis

From a functional point of view, i.e. commercial considerations aside, products are designed to protect, support and/or take over particular human activities, or to extend human capacities. These global notions are specified for a particular design as design requirements regarding product performance together with appropriate mental and physical effort by users, and minimal side-effects such as noise, environmental impact and injuries. In this respect, the question to what extent, for example, data on user activities are relevant in everyday product design, boils down to the establishment of the effectiveness of the information involved for meeting functional imperatives in design requirements. In other words, design-relevance of any type of data requires the negotiated functioning of a design in progress to be responsive to differences in these data when applied.

The actual functioning of a product is an obvious starting point for exploring the significance of various types of data for design. Once product functioning has been charted, the role of users is introduced in terms of both activities and characteristics. Empirical studies carried out by the Applied Ergonomics group at Delft University in recent years serve as source of the data. For the greater part, these studies are user trials focused on the (re)design of an artefact, including video-taping of actions of subjects, at home or in a simulated environment. In addition, human characteristic data have been collected.

As to the notion of ‘everyday products’, it is taken for granted that the distinction between the products featuring in the studies to be discussed, and products which for example are used professionally, may be vague.

The functioning of everyday products

Basically, product functioning is a technical/physical process. For a particular product, this process is set once the actual way of use is given, as well as environmental factors (in this paper, the latter are not studied in detail). A check of a new design against functional parameters, i.e. product performance, the required effort and side-effects, has to account for diversity in ways of use in practice, see the following examples.

Usage dependent product functioning

Designed functionalities may be activated by users at the wrong time, see the operation of the clock-radio in Figure I and the manipulation of the vauum cleaner tube in Figure 2. Figure 3 shows the undermining effect of actual usage for the designed functionality of a new coffee-cream container.

These examples illustrate the indisputable design relevance of actual, as opposed to assumed, usage, i.e. by the consequences of unanticipated variation in use actions. Here, use actions are defined as the external, physical activities of users activating product functionalities. In product operation use actions often involve manipulation, i.e. touching/holding a product or product part, and moving it by exerting force. The

Figure 1 Ways in which some of the subjects (n = 8) switched off the alarm of a clock-radio (Votite et al, 1993): by both operations at the right, the setting of the radio is also

changed, causing the subjects to make extra effort to silence the unwelcome sound source

Figure 2 Ways in which some of the subjects (n=6) held the tube of a vacuum cleaner (Loopik rt al, 1994). By the manipulation in the middle and at the right the mechanical suction-power regulator at the lower end of the tube was inadvertently opened during vacuuming. This was found

annoying as an open regulator considerably reduces the vacuum cleaner’s primary function, while, in addition, the regulator’s position (on the underside of the tube) is out of

sight, so that one must be attentive to an increase in noise as an indication of a change in the regulator’s setting

considerable variation in such use actions is demonstrated once more in Figure 4, showing the ways in which round rotary controls were observed to be operated by physically impaired and non-impaired users (Kanis and Van Hees, 1995).

The presentation in Figure 4 raises the issue to what extent increased detailing of use actions may still be considered as relevant for usage centred design.

The resolution ef‘ use action descriptions. Ideally, the resolution of any description of use actions should be set on the basis of the invariance of the functional parameters for differences in detailing. In the clock-radio study, for example, it would seem significant for designers to know that there are other ways to switch off the alarm than the one shown at the left in Figure 1. In contrast, the distinction between operating with the palm or the back of the hand seems less relevant. In Figure 4 the detailedness of the manipulations is based on the parts of the hand that could be seen to contact the control, and on the movements made by parts of the hand or the whole hand. Each operation involved was successful. This observation as such does not

Usage centred research for everyday product design: H. Kanis 77

Figure 3 Ways in which subjects poured cream with a coffee- cream container (Kanis and Wendel, 1990; Kanis, 1995). In the usage, as intended by the designers, the cover should be removed up to a barrier made by a firm glueing between the cover and the container. Then the cream can be trickled into the coffee through the well-defined opening, thus avoiding spilling the cream. Only a few subjects used the container as anticipated-the figures give the number of times subjects (n= 34) poured as indicated at least once; if shaded, the container is used as demonstrated at least once by the designers (n = 3) showing the usage they intended

Figure 4 Observed manipulations of round rotary controls in several field studies (Kanis and Van Hees, 1995). Number of observations = 100, involving 74 manipulations by impaired subjects and 26 by non-impaired subjects, with 69 controls being operated. Most frequently observed manipulation, upper left: 22 times, least frequently observed manipulations in lower row: 1 time

allow the general conclusion that the resolution adopted would be over-detailed. For, in a number of cases, particular manipulations proved to be carried out in order to avoid strain or extra pain, i.e. to reduce effort or inconvenience. As a matter of fact, for each manipulation in Figure 4 it is possible to imagine a user for whom this operation is the optimum, given physical constraints such as weakness or disability.

These observations illustrate that the setting of a resolution in describing use actions in order to generate functionally relevant distinctions may always be open to question. For designers it seems crucial to be aware of the diversity which tends to characterize use actions.

Product functioning as result of a technical/physical process

Figure 5 summarizes the discussion so far in presenting the functioning of products as the result of the co- occurrence of a particular product and use actions in an environment. It is this co-occurrence with particular product properties and certain environmental factors which specifies human data as use actions. Environmental impact on the functioning of a product would be, for example, any effect of mains voltage variation on the suction of a vacuum cleaner. Sometimes use actions (i) or environmental factors (ii) have little or no significance for the current functioning of a product, for example, a thermostat after it is set (i) or the pouring of cream from a coffee-cream container (ii). On other occasions the functioning of a product may depend heavily on both use actions and environmental factors-think of the operation of a vacuum cleaner on different types of floor coverings.

In their consequences, use actions can be seen as technical/physical phenomena, e.g. emulatable by a robot. In their production, use actions emerge as human behaviour.

Perceptions, cognitions and use actions as user activities

Product functioning extends over time. Consecutive observations, for example, by video, can then be conceived as a string, see Figure 6 for the coffee-cream container used by one subject. When questioned this subject explained that the shape of the container and the tag (Figure 3) led him to tear off the cover at the tag. He

Figure 5 The functioning of a product as the result of a technical/physical process, i.e. as the outcome of the co- occurrence of a product with use actions in an environment

Figure 6 ‘Snap-shots’ from the recorded observation of the usage by one subject of the coffee-creamer cup: holding the container after picking it up (I), removing the cover up to the barrier (2), attempt to pour (3) removal of the cover past the barrier, resulting in a large opening as this subject habitually does with other containers (4) and the emptying of the container (5)

78 tisqe centrrd research everyday design: H. Kanis

had clearly noticed the barrier (picture 2) but did not recognize the possibility of squeezing the soft material of the container in order to pour the cream (pictures 3 and 4). Thus, the actual links between the pictures in Figure 6 are the perceptive and cognitive activities of the user on the consequences of his use actions, and in generating use actions aimed at the product functioning he intends.

Perceptions of users and their ways of reasoning, i.e. cognitive activities, together with use actions, are indicated by the collective term ‘user activities’. Although it appears to be difficult to disentangle perceptive and cognitive activities conceptually (cf. Neisser, 1976), people can distinguish these activities, at least to some extent, e.g. in the coffee-cream container study:

[question] “Did you notice something peculiar in opening the cup?” [answer] “Yes, it was difficult, things were too tight.” [question] “Any idea why this tightness?” [answer] “...?“.

In design, the distinction between not noticing and not understanding may make all the difference. User activ- ities in the operation of products are seen as constitut- ing the sequence {... + use action + perception + cognition + use action -+...}. Actual usage is conceived as a continuous repetition of this sequence, along with the functioning of a product over time.

Figure 7 Operation of a remote control (Vorst et al, 1992): seven out of 10 subjects were unfamiliar with the mute function which can turn sound on and off, two subjects were not aware that they could select more programmes than the three with which they were familiar, two out of three subjects, who knew that up to 20 programmes could be received, had no idea of how to select a programme consisting of more than two figures, while yet another five out of seven subjects, who had video-text installed, were unfamiliar with the controls which accessed it

Productfunctioning accumulating with user activities

The attunement, by feedbacklfeedforward, between user activities and the actual functioning of a product should be seen as cumulative rather than cyclic. This accumulative character both may and may not result in product functioning as designed, see the examples above. The design relevance of perception and cognition, for the realization of product functionalities, is further illustrated in the following examples.

Designed.functionalities not exploited. In a study of the use of remote controls for television by elderly subjects (Figure 7) several functionalities were rendered futile because they were not noticed or their operation was not understood.

Figure 8 Display of the electronic suction power regulator on one of the vacuum cleaners in the study by Loopik et al, 1994: the term ‘AUTO’, which means the position in which the suction power is automatically adjusted to suit the type of floor covering, was understood by one subject to mean the position of the regulator used when cleaning the interior of a car

Designed,functionalities undermined. In the study of the coffee-cream container perception of the cue that the cover is firmly attached to the rim of the container does not, in most cases, prompt subjects to recognize the functionality the designers had in mind, with the consequence that a primary aim of the re-design is missed. Another example is given in Figure 8, from the vacuum cleaner study (Figure 2). The misinterpretation of the term ‘AUTO’ may considerably reduce a convenient and efficient use of the vacuum cleaner.

Imagined.functionalities. Occasionally, users can be seen to assume functionalities that do not exist. An example is found in the vacuum cleaner study, in which subjects tried to operate the electronic suction power regulator by pushing on the pictograms that display the various positions (Figure 8). Another example was encountered in the remote control study (Figure 7). One subject, who did not point the remote control straight at the television, explained that the television did not react because she ‘apparently’ did not push the button hard enough.

Product functioning as negotiated by users

Designed ,functionalities by-passed. In the vacuum The functioning of a product as the result of a cleaner study the subjects were unable to make sense of technical/physical process in Figure 5 can now be the pictograms on the nozzles (Figure 9). With each extended with perceptions and cognitions, in advance nozzle, subjects established the position of the brush by of use actions, and with the effort of any user activity, seeing it, through manual control or from the i.e. in perceiving or figuring out designed resistance when vacuuming, thus by-passing any functionalities as well as in physically operating a reasoned response to the designed pictograms. This product. In Figure 10, this path has been indicated illustrates that incomprehension does not necessarily with dark-grey arrows, and may also run via the prevent subjects from using a product satisfactorily. environment in case this is affected by the functioning

Usage centred research for everyday product design: H. Kanis 79

Figure 10 Product functioning as negotiated by users

individual qualities can only affect the functioning of a product via user activities (Figure ZO). Individual qualities may be distinguished as

Figure 9 Nozzles featuring various pictograms in the study by Loopik et al, 1994. The Illllll-sign on nozzles A and B

means ‘carpeting’, on C ‘brush’, as all subjects interpreted it. The pictogram on A means ‘brush in’; interpretation as ‘brush out’ leaves ‘brush in’ as logical interpretation of the

‘nozzle only’ pictogram on 1% This is wrong: seeing only a nozzle should be interpreted as not seeing a carpet, which is then interpreted as a nozzle working on a hard floor covering and thus meaning ‘brush out’. Correct interpretation of the C pictograms had no effect: in vacuuming, the user can only see the pictogram which is down, i.e. not ‘on’

-human characteristics (including capacities), i.e. relatively permanent qualities, see the (a)-type of data mentioned in the Introduction; -temporary conditions, for instance, the experien- cing of discomfort, of inconvenience or increased effort, and also a particular mood such as being tired, being distracted by bystanders or in a hurry (cf. Weegels, 1996); -experience, as a psycho-motoric condition both activated and constituted in product usage.

User activities versus human characteristics

product, such as the cleaning of a carpet with a vacuum cleaner. The design relevance of noticing and understanding featural and functional cues, as a prerequisite for their effectiveness, is self-evident. However, the nozzle example shows that various flexibilities may be available in the operation which allow users to overcome design deficiencies. By behaving empirically in this case, they avoid cumbersome reasoning, i.e. on a knowledge-based level (cf. Rasmussen, 1986) which would only be resorted to if nothing else works (Kirlik, 1995). In this respect, also the emergence of imagined functionalities can be seen as an indication of the perceptual/cognitive inadequateness of designed featural and functional cues.

Along with the tripartition of user activities, i.e. perception, cognition and use actions, human characteristics (this term is adopted in this paper to include human capacities) are distinguished as sensory, mental and physical qualities.

The light-grey arrows in Figure IO, between the user and the environment, represent additional paths for possible effects of product functioning on users and vice versa. For example, in the clock-radio study, three subjects found the light from the LED display irritating, although it was adjusted to its lowest level. To remedy this, one subject placed the clock so that the display faced backwards, while another subject put books in front of it, making it no longer possible to check the time. Obviously, the significance of such observations for design is beyond question.

Sensory characteristics. In the clock-radio study (Figure I), no relationship was found between the way in which subjects operated the alarm and eyesight (without glasses or lenses on the basis of a visus-map). In fact, each subject managed equally well, both when blindfolded (pitch-dark) or in dim light (3 lux). In the remote control study (Figure 7) the occasional operation of a wrong button was reported by two subjects who were partially blind, and by five of the remaining eight subjects, none of whom had visual problems. Only in a study into the operation of controls (Schoorlemmer and Kanis, 1992) were some tangible connections identified. In figuring out the current position of controls it was subjects who were blind or visually impaired who faced problems due to lack of feedback (three and four subjects, respectively, out of 27). Sometimes blind subjects, groping in search of the right button, would inadvertently operate other controls which required little force for activation. In rotating a control, these subjects appear to manipulate mainly with their finger tips, presumably because these are more sensitive..

Human qualities

Given the relevance for everyday product design of what users do and experience in product usage, it remains to chart and assess the (possible) role of information about who the users are. Whatever this role, it is clear that differences between users as to

These findings may have some impact for design. No such information is available from the referred studies for other sensory modalities.

Mental characteristics. In the remote control study involving elderly subjects, an exploratory attempt was made to relate familiarity of product functions with the

80 Usage centred research,for everyday product design: H. KaniJ

proper answers to casually asked questions concerning e.g. the date, post code and telephone number as tokens for mental capacity. The positive association which was found is not surprising, indicating that the less apparently alert and aware people knew fewer functions. Which functions remained unaddressed could not be related to the answers received. These findings add nothing to the existing insight that product electronification tends to handicap users, particularly when they are older.

Physical characteristics. In the study of the clock-radio, its positioning by each subject had no relationship with the armlength or the reach envelope per subject. In the coffee-cream container study, a fruitless attempt was made to link the way subjects overcame the firm attachment to the forces they were able to exert by pinching between the thumb and the index finger, and in gripping with the full hand (Kanis and Wendel, 1990).

Measurements of force exertion were carried out most extensively in the studies into control operation (Schoorlemmer and Kanis, 1992; Kanis, 1993). Although the tasks for the subjects in these measurements could, to some extent, be seen as a simulation of the actual operation of a control, it appeared that the difficulties users reported in producing the required force were only moderately related to comfortably and maximally exerted forces. Several times subjects succeeded in operating a control which required more force exertion than they had shown themselves capable of in the force measurements. In a number of cases it could reasonably be assumed that extra force exertion was due to the type of manipulation.

The most prominent relationship, it seems, is the one that could be established between the variation in the ways controls were manipulated and the marginal capacity of subjects in force exertion. Figure I1 shows that, over a number of studies in which both physically impaired and non-impaired subjects were involved, most variation occurs within the impaired group. For this group it seems that physical constraints act as incentives to produce types of manipulations to which non-impaired users do not have to resort (Kanis and Van Hees, 1995). The significance of this finding for everyday product design will be dicussed below.

only non-impked q - only imp&red q - impaired and noll-imppi~ uaen

Figure 11 Percentage of (not-) shared control manipulations by physically impaired and non-impaired users

User activities versus temporary user conditions

The examples given above present various critical instances arising from deficient product functioning. It is not surprising that such experiences run side by side with specific use-actions, particularly those aimed at avoiding inconveniences. Such observations exemplify design relevancy. This does not seem to apply to accidental temporary conditions, i.e. conditions which were not related to current product usage, for instance not wearing glasses, which are usually worn, in the remote control trial (Figure 7), and whether physically impaired subjects in the control operation studies had a good or bad day (Schoorlemmer and Kanis, 1992; Kanis, 1993). In these studies no indications were found that effort experienced by subjects, or different use-actions, equated with particular temporary accidental conditions.

User uctivities and experience

In the trials with the clock-radio (Figure I) and the vacuum cleaners (Figure 2) the usage of the corresponding product owned by the subjects was also observed. The trial with the coffee-cream container (Figure 3) concluded with subjects opening a container they had indicated as being familiar. In general, subjects in these studies were found to stick as closely as possible to their habitual way of use (cf. Reason, 1990). In the coffee-cream container study this method of use runs parallel with the finding that subjects who regularly use a particular type of coffee-cream container needed more attempts to open the new container than less experienced users. Here, existing use habits seem to limit the ability to be flexible when confronted by the unfamiliar. This also explains why, of the nine subjects who poured by squeezing the containers, i.e. as designed (Figure 3), all but one had no or relatively little experience in the use of coffee- cream containers. It would seem that lack of specific experience means less constraints in coming to grips with the designed functionality in this case. For further elaboration on this topic see Gelderblom, 1997.

Conclusion

Product functioning as depicted in Figure 10 can now be rounded off with the presentation in Figure 12. Within the user-block, the newly introduced concepts are not ‘arrowed’ to the rest in order to refrain from suggesting all kinds of relationships, since no stance is taken on, for example, the prominent role ascribed to information processing in cognitive psychology, or the focus, in ecological psychology, on the links between users and the perceived environment. In fact, the discussed studies indicate that in particular, human characteristics are not, or only loosely, associated with user activities, and cannot, in consequence, serve for the prediction of user activities. Of note in this respect is that this absence of associations will generally be partially a function of design, i.e. as a consequence of the extent to which designers have properly accomodated for characteristics of prospective users, e.g. on the basis of currently available data, applied as tokens for general boundary conditions (cf. Green et al, 1997).

Usage centred research for everyday product design: H. Kanis

Figure 12 The functioning of a product operated by a user

Design relevance and resolution in user product interaction

In Figure 12, the dichotomy in design relevance of information for everyday product design-low at the right (for human characteristics) and high to the left (for user activities)--can be seen to be bridged by experience and a user’s mood, in as far as these are activated by the user product-interaction at hand. This illustrates once more that in usage-centred design the relevance of data tends to require information which to some extent is specific, or at least specifiable. This returns the resolution issue. Considerable detailing may be needed for relating experience successfully to, for instance, cognitions, in explaining observed user activities as well as in the anticipation of future usage by designers. As a rule, use actions can be detailed meticulously thanks to their ‘recordability’. Troublesome questions of the proper resolution, such as for describing the manipulations of round rotary controls (Figure 4), may eventually be side-stepped. This is illustrated by a study into the use of bicycle racks (Figure 13). This study shows that existing designs, which demand specific user activities, are a source of many and varie’d difficulties in use. The new design (see the model in Figure 13) makes few specific use demands in order to work, i.e. has a low resolution of usage specification. This model allows for flexible use, both in positioning a bicycle and in locking it up. In a user trial with the prototype, difficulties which occurred with existing racks appear to have been largely solved. Note that, in charting use actions, one can not escape from starting with some resolution, which only by hindsight may turn out to be satisfactory.

Discussion

For usage centred design of everyday products, human characteristics seem to be an example of the criteria which Chapanis criticises for their design irrelevance (see Introduction). However, this is not to say that after the present study characteristics of users can decently be ignored completely in design.

To begin with, precaution is needed in generalizing the observations in this study, since the term ‘everyday products’ by no means refers to some uniform or segregated product area, while, in as far as the products featuring in this study do seem acceptable representatives of everyday products, their number is

Figure 13 User trial with bicycle racks (Van der Steen et al, 1996). The functioning of the thin tube of the rack on the upper picture, that is bent backwards at both ends, is not understood and limits the usage for bicycles with a basket or a child seat in front. The anti-theft facility of the second rack was not understood. For the third rack (centre) no cognitive difficulties were observed, whilst the far-reaching specification of use actions (e.g., the positioning of the frontwheel and where to lock the bicycle) goes together with numerous operational difficulties, partly due to adjacent bicycles. The new design is shown by the model on the second picture from the bottom, while the lower picture is from the users’ trial with the prototype

limited. Of note is the restricted duration of usage prevailing in most of the studies referred to.

Next, for setting general boundary conditions, the relevance is not questioned of acquiring, for instance,

82 Usugc centred research for everyday product design: H. Kanis

more anthropometric knowledge as a descriptor of new elements in a population which extend its variability, e.g. by ageing or by the arrival of ethnic groups. Another example is the importance of considering extreme intra-individual variation in force exertion, given a particular way of manipulation, in the adoption of adequate safety margins (Kanis, 1997, as to the design of child-resistant packages). Obviously, differences in human characteristics which were for good reasons accounted for in the design of a currently available product, may for the same reasons remain relevant in case of a new design, e.g. hand dimensions in a redesign of the grip offered by a vacuum cleaner tube.

Yet another token of the significance of knowing particular user characteristics is their possible relevance for the efficiency of subject sampling in user trialling. Generally, user trialling in a design context is not about the prediction of averages and dispersion in quantitative measurement results, but rather about the identification of different types of usage, especially in terms of difficulties in use (Kanis and Vermeeren, 1996). It is of little concern whether a particular use pattern is observed once or twice in 10 subjects. What is important is the use variation. The control manipulation study suggests that this variation can be efficiently charted by using people operating close to the limit of their capacities (Figure 11).

On these occasions, the impact for usage-centred design purposes of data on user characteristics does not seem to thrive on decimal numbers. For the rest, the present study has yielded no evidence for the significance in everyday product design of human characteristics information, i.e. of data at the right in the graphical representation of user operated product functioning (Figure 12). The right-to-left increase of design relevance of user involved data in this figure seems to go along with a decrease in measurability. Measurement of user activities may be questionable, see the difficulty to think aloud cognitive activities in a manipulative task (Ericsson and Simon, 1993). In addition, the analysis of self- reports of subjects is known to be cumbersome, as is the analysis of use-actions, e.g. from video recordings. The measurement of human characteristics, even mental capacities as usually defined, is often comparatively straightforward, at least according to the positivistic ideal of quantities observable ‘out there’, which ideally prevails in many academic circles. This may be a reason why human characteristics attract considerable attention in textbooks on design engineering (cf. Chapanis, 1988). It seems, however, an illusion to expect much good to come from making such data more accessible for usage-centred design of everyday products (cf. Meister, 1989). As shown by the empirical studies discussed in this paper, user trialling can be seen as an obvious way to enable designers to accomodate prospective user activities in everyday product design, rather than descriptive

or experimental research producing isolated data in terms of human characteristics.

Acknowledgements

The author would like to thank Dr L. P. A. Steenbekkers and Dr M. F. Weegels for commenting on earlier drafts, and particularly Professor W. S. Green who also reviewed the English text.

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