I recently encountered unexpected difficulty using an unfamiliar shower. Confronted by a sleek bar without markings it was not clear that twisting the right section of the bar turned the shower on – not powerfully enough, but at an agreeable temperature. Having washed, I twisted the bar again to turn the shower off – only, having used the wrong control, to be burned by hot water. My host later revealed they also disliked the controls – they frequently had to work out how to correctly adjust temperature, and it took them months to discover the hidden button which un-limits flow.

Usability is the process of making products intuitive and easy to use. An intuitive product can be used without conscious reasoning. Therefore, for a “walk-up and go” product such as a three-control shower, the designer must communicate its use through the visual features of the design itself. While usability engineering is a complete process involving testing, feedback and iterative design; rather than a set of rules, let us explore some of the heuristic principles the shower’s designer could have employed.

Affordances are a “property of an object relating to its potential utility, inferred from visual or other perceptual signals”; thus, we pull handles and press buttons. Mappings are the relationship between controls and their movements in the world. When these are ‘natural’ they require no explanation – when they are not, they confuse the user; as happens whenever I use my kitchen hob. Visibility is when functional aspects of a product are visible or suggested, allowing a mental simulation of a product’s operation. It is a lack of visibility that causes us to pull the wrong side of a refrigerator door. Constraints can be both physical or cultural, and limit the possible operation of a product – taps are culturally constrained to tighten clockwise, while sim cards and USB drives can only be inserted correctly. Feedback provides the user with confirmation of an action or the status of a product, for example with a light to indicate power. Gesalt rules are observations of how we see, leading to dominant interpretations of information; TV remotes are visually simplified by grouping buttons using proximity or similarity.

We can now understand why I found my host’s shower difficult to use. The controls were a bar, which did not afford twisting, and did not provide visible clues to its operation, such as a lever or indents for my fingers. Visibility of the power limit was also ignored. Showers have poor feedback due to the time delay of a temperature change – after which an error could freeze or scald the user; such an error should be unacceptable, but was instead likely due to no differentiation in style between the temperature and power controls. Additionally the mapping between twisting the bar and affecting a change was neither natural nor used the cultural constraint that a clockwise turn reduces flow, or that red indicates hot and blue cold.

Though designers are becoming increasingly aware of the importance of usability, there exists an unfortunately great number of confusing products, my host’s shower being just one example. A quick and inexpensive usability study would have allowed me to intuitively operate their otherwise exemplary product, and contributed to their reputation.