Multiple mental models are bad, or ‘What a toilet can teach us about UX design’ 

First published on Medium on Mar 22, 2021

My toilet confuses me which makes me feel not very clever 😞 I never know which is a big flush a which is a little flush.

This is the flush mechanism. It’s nice and smart and looks great in our bathroom, it’s a really nice design. But does the big button do a big flush or does the big button do the action you want most often — little flush? There’s no clue on the flush mechanism itself. That’s less good design.

As I know a bit about design theory the answer is obvious — Fitts’ law says “the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target.” What that means is bigger buttons are easier to press; so the designer must have intended the big button to do the most common action — the bigger button does a small flush.

But seeing as I also know a bit about design theory the answer is even more obvious. The designer is using a metaphor; the size of the button represents the size of the flush — the bigger button does a big flush.

Oh wait… Maybe knowing about design theory doesn’t solve this problem. (Perhaps doing some user testing would have helped though🤯)

Designing products that allow users to have more than one mental models in their heads at the same time is bad. Multiple mental models are guaranteed to to cause confusion and probably make the user feel pretty silly too. Good design helps users discard mental models as well as creating new ones — clarity comes from the absence of alternatives as well as a having distinct path forward.

If you haven’t been to yet, it’s a great repository of ‘laws’ of interaction design that help users form mental models of the interaction

A fun outcome of Fitts’ Law with interfaces that use pointers is the idea of infinite edges