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Human Factors of Controls

Marc Green

The goal of human factors is to create artefacts that are consistent with innate human limitations and predispositions. Perhaps the best way to understand human factors is by examples of designs that cause error because they fail to allow for human capabilities. In some cases, the limitations are obvious - people cannot see when the light is too dim, objects to small, etc. In other cases, the limitations are more subtle. Here are some examples of poor human factors from relatively "simple" everyday objects.

1. Poor Light Controls

Here's the light switches on my upper hall landing. One is the upstairs overhead light, one one the downstairs overhead light and the other is the bathroom. In spite of living in the same house for 17 years, I still frequently hit the wrong switch, turning upstairs on when I mean downstairs and vice versa. There is no logical relationship between the switches' layout and their effect on the environment.

A properly designed switch would look like this. The upper and lower hall lights would be mapped to their actual locations in space and the bathroom light switch would be obvious due to different orientation. This is an example of the need for "stimulus-response compatibility," the establishment of a natural connection between the way controls look and the the way they act.

2. Poor Electronic Device Control

This is the remote for my old Mitsubishi VCR. The major controls are located in a ring. The "stop" button is a tiny dot in the upper right. Naturally I push "pause" when I mean "stop" much of the time. I also confuse the "eject" and "stop" buttons because their locations are completely arbitrary much like the positions of the hallway light switches. People think of actions in pairs - stop is the opposite of start, so that's where it should be located. Further, the most commonly used controls should largest and easiest to find - "stop" is hard to find and hard to push because of it's size. This is especially important for TV and VCR controls because people do not typically look at them, relying instead on felt position.

3. Poor Elevator Control

This picture shows the elevator from a local shopping mall. I have used this elevator many times and have both pushed the wrong button many times and observed other people also making errors. It goes from the single parking level to the single floor with stores. There are two buttons, parking level (P1) and ground (G). (The third button does nothing as far as I can tell.) For such a simple device, it has many human factors errors.

First, the buttons are labeled incorrectly. "G" is for "garage" which means parking, so I often errantly press G get to the parking level. Why label the other button P1 if there is only one parking level? Moreover, the placards are not highly conspicuous, and they don't match the buttons. "Shopping Concourse" translates to the "G" button and "Covered Parking" to "P1?" There is no connection between the signs and the buttons.

Second, the buttons are misaligned. Elevators travel up and down, not side ways. The buttons should therefore be aligned vertically with P1 below and illuminated when in parking and G on top and illuminated when the elevator is on "ground." (Actually, it would be better design to illuminate the button for the floor that you want to reach than the floor that you are on. The lighted button is the one that draws attention and says "push me." Unfortunately, this is not standard and it's better to stick with convention most of the time.) That would remove the need to even look at the labels. It's another version of the problem with the light switches - the control layout fails to be consistent with the actions that they perform.

Third, there are too many buttons. There should not be two buttons. If you are on the ground level, the only place you can go is parking. If you are on the parking level, the only place you can go is ground. If there were only one button, the user could never make a mistake. The goal of human factors is to head off error before they occur.

4. Poor Appliance Control

I recently burned dinner and destroyed a pan because I errantly set the heat to high rather than to medium. I blame it on the stove's badly designed controls. The picture shows two of the four controls on my fancy new stove. Three of the 4 burner controls are similar to the top one shown in the picture. The "Max" position is at about one clock and the "Min" is at about 11. To set the burner medium heat, for example, you turn the arrow to point down, somewhere around 6 o'clock. The lower control is the oddball. Note that it has two sets of Min and Max markings. This is a fancy burner that has both a small and a large heating element, and the more complicated control does double duty, letting you set either. The two markings on the left refer to the smaller diameter-heating element and while the two on the right a larger one. Most important, turning the arrow downward is likely to set the burner to MAX, not medium as on the other burners. That's exactly what I did. Wanting a medium setting, I merely flipped the arrow downward as I do on all the other controls.

The human factors error here is making controls look the same but work differently. But there are labels you say? One basic human factors rule is that people quit reading labels after extended use due to cue generalization. Never depend on labels to guide behavior and to prevent errors.
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