Recently I asked the question “Should you optimize mobile experiences based on individual handedness?” on the UX Stack Exchange board. This question came to mind while looking at this mock-up that UX.SE user abbood created:
My immediate thought was that the up-vote control was on the wrong side. Why? Because my feeling is that up-votes happen more often than down-votes — something not interesting is not worth spending a click on — and on mobile the primary action (up vote) should be on the left. But I realized this view was quite self-centric: I’m a lefty. For a right-handed person having the primary control on the right probably makes much more sense, I thought.
Applying Fitt’s Law to touch devices
Justin Smith wrote an excellent article on the application of Fitt’s Law on touch devices. In his article he argues that Fitt’s Law applicability to mobile experiences depends on the way the user is holding the device. If she doesn’t need to change the way she holds the device the law applies. For example, when using the device single-handedly in portrait mode. It also applies when holding the devices double-handedly in landscape mode (assuming you are able to reach all parts of the interface, which depends on the size of the device). However, when holding the device with the one hand and controlling it with the other Fitt’s Law doesn’t apply, as you now need to take other variables into account.
In my experience the way a user holds a device depends on how she interacts with it. When drafting an e-mail it makes sense to use two hands. However, when doing a a check-in on Foursquare it makes sense to only use one hand. The interaction time is very limited and shouldn’t require a lot of movement. On top of that we should also consider the context: the user is probably entering (or has just entered) a space and is doing multiple things at the same time. Looking around for familiair faces, grabbing a seat or shaking someone’s hand (regardless of how rude that is).
Foursquare’s primary action: the check-in
Foursquare offers a lot more functionality than merely doing a check-in, but checking in is still the primary action. What’s more: it’s also the action that usually takes place in the context I outlined above (while being in the middle of some other activity).
Therefor the most important action for a Foursquare user is to reach and tap the check-in button in the interface. In the image on the right I’ve highlighted the target area. As you can see it’s in the top-right corner.
This shouldn’t present a problem for right-handed users holding their iPhone in their primary hand. Although it’s not easiest target to reach with just a thumb, it’s certainly possible.
However, when holding the device in your left hand and using your left thumb it’s simply impossible to reach the target. Let’s take a look at the thumb reach of the left hand drawn on top of the Foursquare interface.
Left-handed thumb reach in Foursquare
In the image on the right I’ve used green to illustrate the primary thumb reach area, the area that is reachable without requiring a lot of stretching of the thumb.
I’ve highlighted the secondary thumb reach area in orange. This part is still within thumb’s reach, but it requires some stretching to cover it. Anything outside these areas is simply impossible to reach without moving the position of the device within the hand or using a second hand (or having absurdly long, flexible thumbs).
What becomes clear when looking at this illustration is that it’s impossible to reach the check-in target single-handedly with your left thumb. I can confirm this from my own experience: doing a Foursquare check-in requires me to either control the device with my right hand or use two hands: one for holding the device and one for tapping.
Note: these are just approximate illustrations, actual thumb reach may differ from person to person and also depends on the form factor of the device.
Is this a problem? Partially, yes.
Given the fact that 10% of the world population is left-handed my initial thought was that this would be a problem. Ignoring 10% of 7 billion people doesn’t seem to make sense. But that’s not necessarily the case.
In another answer to my UX.SE question adrianh linked to a very interesting article published on UXmatters: “How Do Users Really Hold Mobile Devices?“. In his article Steven Hoober shares the results of 1,333 observed interactions with mobile devices. The finding that surprised me the most? That in 33% of all single-hand interactions the left thumb was on the screen. This doesn’t match with the left-vs-right handedness distribution at all.
One other thing that stood out was that users continuously switch the way they hold their phone. This suggests that the order of an interface has less impact on lefties than I expected. However, we can only guess if the observed behaviour comes naturally or if users behave this way because they are adapting to the interface. Does the interface drive behaviour or is it the other way around?
In either case, optimizing an interface for usage in either single hand or any other configuration — regardless of handedness — should not be ignored.
I initially asked my question based on the premise that most device interactions happen single-handedly and using one’s primary hand. Steven Hoober’s findings debunk this, or at least partially. But that doesn’t mean that you should accept this on face value and ignore handedness in your interface design.
Adapt to individual handedness
My first idea was to detect the handedness of the user and dynamically adapt the interface to this. Detecting handedness is probably very hard to do, so maybe it should be a manual setting (no pun intended) within the app’s or platform’s configuration. However, if we look at Steven Hoober’s findings that would hardly be necessary: users continually switch the way they hold their device and a lot of the single-handed interaction isn’t performed with the primary hand. On top of that this method would present all kinds of difficulties like interface recognition problems.
Choose a neutral interface order
UX.SE user Benjamin Malley presented a different suggestion. Instead of choosing between optimization for either left-handed or right-handed usage you could also choose for a neutral solution. Compare the following two interfaces:
The interface on the left shows the iPhone’s Lock screen. In this interface the user needs to slide the bar from left to right to execute the unlock.
The interface on the right shows the “Power down” screen on Windows Phone. The user needs to slide the bar downwards to confirm powering down.
In both situations a slide action is required to confirm the action and to prevent accidentally executing the action. However, Apple’s interface seems to have a bias towards right-handed usage. Sliding from left to right is harder to do when holding the phone in the left hand than it is when holding the phone in the right hand.
Microsoft decided on a neutral solution: the slide down isn’t harder when executed with the left hand instead of the right.
Prototype, test and refine your interface
There’s one recommendation that can safely be given: prototype, test and refine your interface. This advice applies to almost all UX challenges that we confront, and this one is no stranger.
There are various ways to test your interface. User-testing in a controlled setting by recording the mobile interaction is an obvious way to do it. But you could also consider quantitatively A/B testing your interface. Design an alternate version of the interface and serve it to a sample of your user base. You could consider KPIs like how long it takes for users to reach and tap the moved button, or how often mis-clicks happen, etc. Doing A/B testing of interface lay-out maybe hard when you’re doing native apps, but when you’re doing web apps it’s certainly possible (another win for the web :).
Regardless of the used testing method, one thing is clear: you can’t replace testing on actual devices. It’s impossible to reliably test element reachability on paper sketches. And apart from the reachability of the controls in your interface there’s also the issue of fingers overlapping the screen. You want to make sure that users don’t hide essential information on the screen just because they are interacting with your interface (especially in interaction-rich contexts). So, you absolutely need to prototype and test on real devices.
We’ve learnt that the hand in which users hold their devices doesn’t need to match with their individual handedness. But, interfaces can definitely be much harder to control when using the left hand instead of the right (or vice versa). You should not ignore this. Aim for neutral solutions and make sure to prototype, test and refine your interfaces.
Many thanks to the fantastic people at UX.SE who share their knowledge and experience with the UX community for nothing but reputation and badges. Special thanks to abbood, adrianh and Benjamin Malley for sharing their insights.