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One can touch in between the app icons on an iPhone without launching an app. Given Fitts's law, that "the time required to rapidly move to a target area is a function of the distance to the target and the size of the target" (Wikipedia), would there be any reason, aside from technical constraints, to avoid maximizing the size of the touch areas?

Of course, the touch areas should not extend so far as to conflict with user expectations, but the app icons on an iPhone are already close together. It seems that eliminating the dead space between the touch areas would only enhance usability.

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2 Answers 2

Leaving space between the active areas reduces the chance of launching the wrong app.

Though it also reduces the chances of launching the right app, the trade-off is worth it, especially when you consider the costs associated with not launching the right app on the first press (just try again a split-second later) and the costs associated with launching the wrong app (wait for the wrong app to load, exit, then try again).

You could even calculate how wide the "dead" area should be based on the probabilities, for different widths of the dead area, of

  • not launching any app (hitting the dead area), and

  • launching the wrong app (which can still happen even with a dead area)

and the different expected times to recover (or some other measure of annoyance) due to each of these two things happening.

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I would argue that Fitt's law when applied to the touch screen of a smartphone has slightly different results than when applied to a mouse and desktop screen situation.

A smartphone divides into far fewer discrete active areas than a screen, a fact that is compounded further by the huge difference between interaction tool (mouse versus finger) as well as occupying a much smaller space, so there is much less room for Fitt's Law to return a large value. Once focused on a smartphone screen, while using a pointer as large as a finger, there is a lot less room to actually move, both when considering the physical size of the screen and when considering the size of the grid of active areas.

Consider this: if you were using a mouse and screen to point at a set of icons that were same size as smartphone icons and filled the screen and you were asked to click a random one of those icons. Another consideration: if the desktop top screen were filled with the same number of icons as a smartphone (say 20) but scaled to fill the screen. In the first case Fitt's Law would return a high value and the interface would have issues. In the second case Fitt's law would produce a low number. You could also imagine a smartphone screen sized area with 20 icons in which the mouse pointer was confined and think of those results too.

Also, I think smartphone touch screens also react to areas of higher contact whereas a mouse is pixel precise.

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