Are Small Hit Areas Easier To Hit?

The question of “fat finger” mistakes on touch screens came up in conversation the other day, together with the idea of making targets large to avoid this. At first, it seems sensible to make hit areas for controls on mobile devices as large as possible. But it was pointed out that, counter-intuitively, smaller hit areas can decrease fat finger errors.

That is true to an extent, but as with all things HCI, it’s only a part of the picture. So I thought I’d try to summarise the issues, and to recap as a reminder to myself, if nothing else. What follows is the relevant parts of this excellent article by Steven Hoober, condensed for reference.

First, some definitions: a “touch target” is the area that appears to the user to be touchable. A “hit area” is the area of the screen that will actually respond to the user’s touch.

The absolute minimum for a reliable touch target is 6 millimeters, but this is suitable only when users are relatively still. If users may either be moving or distracted, an 8-millimeter target is better. There is never any need to exceed 15 millimeters for a touch target.

A touch target’s visual design drives users’ expectation of the hit area’s size. If somebody can reasonably expect an entire button or other element to be tappable, then it should be. The most common design error here is to make the difference between the hit area and apparent touch target too large.

Once you have a device on the screen that has a suitable touch target and hit area defined, you need to check for “interference”. This is when the distance between the hit areas on centre is less than the recommended 6mm. Too close, and people are aiming for one touch target but “fat fingering” another hit area by mistake, because it’s too close.

In most cases, you need to either move the devices apart to avoid interference, or reduce their hit area size – and hence the observation that smaller hit areas decrease fat finger errors. It’s true if it decreases interference, but only if the resulting hit area is 6mm or more. Any smaller and errors will then increase despite a lack of interference. Of course, sometimes you can’t move the targets because of this, in which case you need to look at other ways of mitigating the risk (move risky things away from each other, or have an undo, for example).

For the curious, the 6mm recommendation is from a number of HCI studies that show that people will be able to hit an area of this size with the tip of their fingers 95% of the time. This in turn is related to a concept from ballistics called the circular error of probability (CEP), if you want to Google it. So “CEP95” is what we are talking about.