People are never going to be able to precisely click your target. There’s always inaccuracy. But you can account for it in design. 

For capacitive touchscreens, which is what almost everything uses today, the electrical conductivity of your finger is sensed by the screen. In part, this means, that what is always detected is the centroid (or geometric center) of the part of your finger that gets flattened against the screen.

The phone can’t sense how big your finger is, how hard you pressed, how big the contact area is, which way it is facing or anything. All it gets is a point that it assigns to be the touched coordinates. And that point is never, ever perfectly aligned. 

Figure 5—Logging click points on an actual target, like this one, shows the inherent inaccuracy of tapping. Touch targets must be made big enough that almost all taps will result in the expected action. 

Accuracy is never perfect, but has to be considered relative, and is defined with the Circular Error of Probability, or CEP which is just a mathematical representation of how much you miss a target. The touch target sizes I suggest are based on R95, which means 5% of taps still fall outside the area. 

When we accept everything is imprecise, we stop calling these errors, and refer to tolerances instead. We need to plan for imprecision and problems as part of the process. Be sure to provide the largest practical touch targets and don’t just code the word or icon as a link inside a non-selectable box. 

Look around and you’ll see this is a known best practice. The Google drawer menu isn’t as small as it appears, and it also opens when you on select the icon or title. Lots of hybrid apps and websites don’t notice this, and code it wrong, giving the “hamburger menu” a bad reputation for being too small.


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