One of the principles of interaction design as laid out by Don Norman is constraints. It is easily overlooked, because it restricts interaction instead of facilitating it, and is quite often ingrained in designed systems and taken for granted. In short, constraints guide you toward the ideal way of interacting with objects by eliminating the possibility of unwanted interactions.
I shall be talking about physical constraints, which are primarily implemented through limiting Degrees of Freedom (DOF), in which you have an object that can only go a set number of ways, like a knob can only be rotated, not moved, and a switch only goes up or down, not sideways or rotated.
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| Knobs and Switches. I own an Orange amp, and it's one of the best things about interfacing with it. |
Part of why physically constrained interfaces work on automobiles, through knobs, switches, and even the gear shift itself, is because it takes the need for precision away from the hands of the user, and is baked into the design itself. Just imagine if you had to precisely line the gears up yourself in a manual transmission car. Having to take your attention off the road can be costly. This is also why I am not a proponent of screens as car interfaces, like you might see in Tesla cars. Even something as simple as adjusting the air conditioning in a car can be made more difficult without the use of physical constraints. Imagine: A knob only goes one way, and you don't even need to look. A screen, however, lets your finger go anywhere, even off-screen. How do you know you're tapping the right on-screen buttons if you don't look? (Granted, these self-driving cars may assume you aren't focused on driving...)
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| It's infinitely useful how there are grooves for the stick to line up into. |
Restricting Range of Motion
There are many ways to engineer physical constraints into designs, as illustrated above. I would like to examine Range of Motion (ROM) as another oft-underlooked avenue that complements DOF constraints.
If DOF defines how many ways you can move an object, ROM defines how near or far said object can be moved. For example, some rotating joints can all be said to have 1 DOF of rotation, but ROM could differ between objects that freely rotate about the axis (like a wheel), or with limited range of angles, like a volume knob that only goes from 1 to 10.
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| These go to 11. |
Some examples occur so often in daily life that you might wonder why such design isn't more common. For example, soap dispensers. Have you ever gotten a haircut, and taken a shower after, only to realise you've pumped too much shampoo out for what little hair you had left? That's exactly what I'm talking about. While I don't think we actually need soap pumps that gauge just how much shampoo you need, I think there's room for design consideration here.
To illustrate, suppose you need 5mL of shampoo to lather your hair appropriately, and suppose your sister needs 10mL of shampoo to lather her hair, seeing as she has more hair. One shampoo bottle dishes out 10mL of shampoo in one pump, so your sister presses the pump to get all the soap she needs in one pump, while you modulate your press, making sure not to use too much force, and stopping halfway, trying to estimate 5mL each time you use it. Another shampoo bottle has a shorter pump, and gives 5mL of shampoo in one pump. You simply take one full pump to get the soap you need, and your sister takes two pumps, and every time you shower, you know you have the amount you require.
I should note: some households (like mine) find it useful to dilute the soap such that a full pump gives you the appropriate amount, now that the concentration has been lowered.
With that illustration, it's obvious which is the preferred design for this scenario. There are many like this. You may have seen illustrations on your toothpaste tube recommending you to use a pea-sized amount. If I'm not wrong, it's only a recent development that they started recommending toothpaste amounts. In any case, I believe it's the case that many advertisements showcase excessive use of products (soap, toothpaste) to nudge the user to use more product. If nothing else, this has an economical effect, as you cycle through products more. "Pea-sized" is ambiguous, but if toothpaste tubes somehow squirted the recommended amount each time, that would solve the problem, even for people unaware of the "pea-sized" recommendation. Understandably, such designs come at a cost to the product manufacturers.
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| A figure from the above linked study. Consistency is harder to achieve when human error and ambiguity are factors. |
Another scenario: toilets
Another occurrence in which design can be considered is with bidets. Many times I have encountered bidets with far-too-high water pressure, and I say with minimal exaggeration it could clean the gunk off my skin and take the skin with it too. Many such bidets use a trigger, and in cases where the valve is inaccessible, the solution could be to press the trigger half-way to modulate the amount of water coming out... which as you might imagine, is a difficult task to do. Should the solution be to make it such that a fully-pressed trigger gives the right water pressure – a gentler water pressure?
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| When I write, I have these in mind, which are widespread throughout Southeast Asia. |
But then, a problem presents itself. I also use bidets for cleaning. If it isn't aimed at myself, it's aimed at stubborn stains and dirt in the toilet. I'd then want a higher water pressure. The ideal solution to this, in my opinion, lies in adjusting the flow of water through the valve rather than through the trigger. That, or having multiple settings, like with some shower heads. The dual-flush toilet is one such invention that encapsulates this design idea (sort of). If you only had one flush button, and you wanted to save water, you'd probably press it lighter or shorter if you only had liquid business to dispose of. Why bother with that? Have a big button and a small button you can push all the way, and save yourself the trouble of having to be precise.
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| Adjust the water flow here, and then press the trigger fully. |
On a side note, the place I'm currently staying at has a very weak flush. Big stuff takes one or two flushes... so in designs like these, it is important to investigate requirements or the usage falls short of expectations. Flushing the toilet isn't like pumping soap. Two small pumps of soap effectively gives you double the amount, whereas two weak flushes may still not get the job done like one strong flush.
Conclusion
Where usage requires precision, the obvious choice is to enable precision through design, rather than expect the user to be sufficiently skilled or conscientous. In many cases, the consequences could result in product waste at minimum, or be significant to the point of having an effect on one's health.
We should look to designs like dual-flush toilets, which bakes the act of "modulating amount of water used" directly into design instead of placing the responsibility on users' hands. As an effect of widespread use (these are the de-facto default in Singapore), I have no doubt that many liters of water are saved daily. Likewise, we could reduce needless consumption of soap, toothpaste, and perhaps there might be other applications of ROM constraints in design beyond portion control which I have not explored here.
