Certain parts of the screen are either harder to touch, take more time to achieve an adequately precise touch, or both. Since much of the screen offers the same level of precision, I had previously more or less averaged touches across the viewport for my guidelines.
This wasn’t crazy, because no other guidelines that I am aware of account for accuracy by screen position. But several research papers do, with enough precision and consistency that the results are quite believable. The results are simple: edges are hard to hit.
Well, not really hard to hit in the classic sense—causing people to avoid them—but are more difficult to hit accurately, take more time to hit accurately, and cause users to have reduced confidence in their ability to hit them accurately. It turns out that users actually realize they are bad at targeting the edges of the screen.
And I do mean edges. There are such small—and inconsistent—variations in accuracy and speed between left- and right-hand users that it is safe to say that both the left and right edges of a screen are equally hard to reach for all users. However, the top and bottom edges are much worse than the sides...