Sequential Presentation

Compared to desktop solutions, user interfaces in mobile phones are pretty simple. There are far fewer features, even though the number is rapidly increasing. User opportunities to manipulate user interface (UI) objects are essentially more limited than with PCs. With only a couple of keys, the user can hardly select a wrong one. Where is the challenge? Aren't the problems just a fraction of those with desktop interfaces, a small subtask for a desktop application designer? Is mobility an excuse for us to distinguish our position among the mainstream desktop user interface design experts?

Mobile phones get carried around. For some extreme users, terminals follow wherever they go. For the rest, terminals are at hand for a significant proportion of their active time. Phones reside in pockets, in handbags, on belt clips, and in holsters. If these devices did not accompany the user, the main benefit of a mobile phone would be lost-it wouldn't provide immediate access anywhere any more, it wouldn't render the user reachable, it wouldn't be so personal. It would, in short, forfeit its bid to be our primary personal communication link to other people and to services.

The most obvious engineering solution to ensure portability is compact size. Smaller terminals allow more freedom in the way they are worn. The negative impact of decreasing size is also obvious-it influences the sizes of physical user interface elements. The total surface area of the terminal limits the number and size of buttons. The same applies to the screen. Given the limits of human vision, the amount of data that can be presented on the screen at a given time is very limited.

The majority of UI design and usability experience as a whole has been gathered from desktop interfaces, which are characterized by direct manipulation. Direct manipulation interfaces provide permanent representation of objects of interest, the physical operations that can be performed on those objects, and immediate visual responses to such manipulations, as displayed on those same objects. Desktops have Web interfaces with numerous options labeled by texts, most of them long enough to resemble natural language, or by icons, thumbnail images organized-when well designed-into visual structures that link relevant topics to support the user's search strategies. The difference in designing UIs for desktop environments versus phones is about quantity; desktops can accommodate more. However, when we delve a little deeper, the difference in quantity turns into a difference in quality. The logic of PC interfaces does not scale downward. Small interfaces are essentially different from big ones.

Parallel representation, in which plenty of options are displayed simultaneously on a sizable screen, turns into sequential representation on a small screen. You have to browse through options one by one because you can see at best a few at a time. We know that on a 17-in screen the user is able to navigate dozens of interactive items. Learning to use these interfaces fluently depends on the user's ability to match locations on the screen with related features. There can be a lot of mere stuff on the screen, but fluent users are easily able to pick out the ones they use most often from the rest. What is essential about parallel representation is that I may use different items for my purposes than you, and yet the presentation allows both of us to develop personal skills to match personal needs. For you and me, then, the UI becomes two different interfaces because we have learned to use it in different ways. Still the interface is the same from the manufacturer's point of view. The manufacturer's responsibility is to provide a set of features, show them all, and let the big screen and the user select the priorities.

Here I admit to exaggeration. On a desktop UI features are prioritized, albeit not very heavily. The user may, and often does, personalize the interface tools, but is able to develop personal styles of use even without customization of any sort.

A mobile user interface presents just a few items at a time. Users can't necessarily hit their selection right away, but must expect to browse the menu in the order that selections are presented. Everybody browses the menu in the same order. An easy way for users to opt out would be to avail themselves of shortcuts to their preferred items, or to tailor their menus as they wish. However, providing a means of tailoring does not remove our responsibility to provide usable defaults. Mobile phones are consumer goods, and we cannot expect the user to configure them. So, aside from knowing what kind of features to provide, we need to know-for each object in each situation-the most likely next few functions. It's easy enough to provide immediate access to a couple of options, but any user who is repeatedly forced to browse through, click by click, to the seventeenth most likely selection will be beyond frustrated, whereas with a desktop interface with 40 icons in plain view, this user would never even notice that the first choice was in position 17.

Once we focus in on user needs, knowing the user allows us to select the most relevant features for most situations, which in turn enables us to present compact and portable designs. (See Fig. I.1.) It also enables us to provide cost-optimized solutions for people who don't need all the bells and whistles. Mobile sequential interfaces are fragile to bad design. We cannot provide all the options without reference to the essential ones. Mobility via limited output capabilities and sequential presentation leads inexorably to the requirement for feature prioritization.