Sensory Capabilities and Demands

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Table 7-12 shows measured demand values (forces) for activating or manually handling interface features on the four products. These included weights, linear forces and rotational forces. The product demand values were plotted as red vertical lines on capability/demand graphs as elaborated in the next sections.

Table 7-12 Product demand values for motor actions

Motor Demands Product Action Type Force/Torque

Push Buttons Clock Radio Finger 3.3 N

Push buttons Mobile Finger 2.9 N push force

turn the control dial Blender Finger 0.236 Nm clockwise,

0.177Nm anticlockwise

push the PULSE button Blender Finger 9N

slide the on/off switch Vacuum Cleaner Finger 13 N

Lift chassis Mobile Grasping 0.82 N weight

Open cover of the blender jug Blender Grasping 19.8 N

lift the blender jug Blender Grasping 19.3 N

pour from the blender jug into the cup

Blender Grasping 19.3 N

pull the plug and cord out Vacuum Cleaner Grasping 14.7 N

push the cleaner forward Vacuum Cleaner Grasping 15 N

pull the cleaner backward Vacuum Cleaner Grasping 30 N

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Table 7-13 gives the values of the correlation coefficients (Pearson’s r) for each action.

Values in this table (and all following tables with correlation coefficient values) are

highlighted in red if the absolute value of r is strong (0.7 to 1) and in yellow if moderate (0.4 to 0.7). Values that are not highlighted are to be interpreted as weak (0 to 0.4). Each graph plots the contrast capabilities of participants at a particular size measured in LogMar against the rated difficulty in seeing text of a given contrast (at the same size). For example, the contrast demand of the clock radio digital display text is 0.85 (in Log Contrast Sensitivity units) at the size of 1.38 LogMar. A linear model accounted for a significant amount of the variance for actions involving reading textual features on the products.

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Figure 7-28 Eight capability-demand graphs for seeing textual features on the four products

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Table 7-13 Correlation coefficients for seeing textual features

Product Action Correlation Coefficient

Clock Seeing numbers on the digital display -0.782

Clock Seeing button labels -0.766

Blender Seeing numbers on the rotary control -0.737

Mobile Seeing texing on the screen -0.688

Vacuum Cleaner Seeing text on power switch -0.683

Mobile Seeing button text -0.461

Clock Seeing 'PM' text on the display -0.424

Vacuum Cleaner Seeing beater bar button text -0.352

Figure 7-28 shows fairly strong negative relationships for seeing button labels r(16)=-0.766, p< 0.01 and reading numbers on the digital display r(16)=-0.782, p< 0.01 of the clock radio.

Strong relationships were also observed for seeing the screen text on the mobile r(16)=-0.688, p< 0.01, seeing the control numbers on the blender r(16)=-0.737, p< 0.01, and seeing the power switch text on the vacuum cleaner r(15)=-0.683, p< 0.01. However, other relationships showed weaker correlations including seeing the ‘PM” text on the clock radio display r(16)=- 0.424, p< 0.05, seeing the button text on the mobile r(15)=-0.461, p< 0.05 and seeing the beater bar button text on the vacuum cleaner r(14)=-0.352.

In general, participants 1, 3, 9 and 19 tended to score difficulty much higher than other participants. Participants 1, 3 and 9 had comparatively low cognitive scores compared to the rest of the sample, which may be a factor in the way they used the difficulty scale to make judgements. Participant 19’s cognitive scores, however, were not as low as 1, 3 and 9. The graphs show, in general, a vertical spread to difficulty ratings at high levels of capability. This indicated that participants had different levels of difficulty even though their capability levels were roughly similar. However, the trend of correlation lines indicate that rated difficulty does increase as a participant’s capability threshold approaches the product demand line. In addition, participants whose capability fell to the left of the demand line rated difficulty as being maximum or near maximum.

7.3.6.2 Seeing Product Controls and Features

Some significant linear relationships were also found for actions involving seeing product controls and features, as shown in the 6 graphs of Figure 7-29 over the next three pages and in Table 7-14. Seeing the cord retractor button r(14)=-0.771, p< 0.01, the handle recline button r(15)=-0.721, p< 0.01 and the power switch r(15)=-0.749, p< 0.01 on the vacuum cleaner all produced fairly strong linear relationships.

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Figure 7-29 Six capability-demand graphs for seeing interface features on the four products

However, other cases showed no significant linear relationships, for example seeing the buttons on the clock radio r(16)=0.051, seeing the buttons on the mobile r(16)=-0.197 and

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seeing the control dial on the blender r(16)=-0.197. Similar to the graphs in the last section for seeing text, participants 1, 3, 9 and 19 tended to score difficulty much higher than other participants. The graphs also show a similar vertical spread to difficulty ratings at high levels of capability. Unlike the last three graphs in the figure for the vacuum cleaner, the first two graphs show participants with low difficulty ratings, even though the points lie to the left of the demand line. The slope of the first graph is also positive with r(16)=0.051 given the vertical spread of difficulty scores. This may be due to the use of the contrast sensitivity measure for text as an approximation in place of a spatial contrast sensitivity test using sinusoidal gratings (Colenbrander, 2003; Fletcher & American Academy of Ophthalmology., 1999; Schiffman, 2000). It may be that the approximation holds for the vacuum cleaner when the distance of features from the user when in operation is approximately 1 – 1.5m. The perceived size of the feature would then be comparable with the stroke width of the text used in the capability measure.

Table 7-14 Correlation coefficients for seeing product controls and features

Product Action Correlation Coefficient

Vacuum Cleaner Seeing cord retractor button -0.771

Vacuum Cleaner Seeing power switch -0.749

Vacuum Cleaner Seeing handle recline button -0.721

Blender Seeing control dial -0.348

Mobile Seeing buttons -0.197

Clock Seeing buttons 0.051

7.3.6.3 Within Product Comparisons

For the clock radio, the difficulty ratings for reading text on the LCD followed a similar pattern. There was a wide vertical spread of points indicating that different participants of similar capability levels were finding different degrees of difficulty with the task. The difficulty ratings for reading text on the clock radio buttons were less vertically spread out however, resulting in a better linear fit. The mobile showed the opposite pattern where text on the screen had less of a vertical spread of points than reading text on the button chassis. There was a difference between the clock radio LCD and the mobile phone display in that the clock radio LCD was light text on a dark background, while the mobile phone display was dark text on a light background. This may have been a factor in the perceived difficulty of reading on both screens. The blender and the vacuum cleaner followed the vertical distribution of points for visual actions within the task.

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