The Carry Principle

From MobileDesign

Mobile phones differ from computers in that many - if not most- mobile phone users always carry their device with them. This simple concept, which we call The Carry Principle has implications for device, application, and service design.

The mobile phone is:

• Small, since users will not carry large devices. Thus the primary screen will always be small, although some flavor of projection can be used as a backup screen.
• Multi-purpose, since users will not carry a variety of single-purpose devices full time. Thus only a small set of primary features will be readily usable, and others will require extra button presses, navigation of menus, and so forth. While making every function available with a single key press is technically possible, few people will want to carry around a 100-key device. The camera on the Sanyo MM-7500, for example, requires a key press for the suite of camera functions, and then the selection of the first item in the list.
• Personal, as an always-carried device is always carried by one person. The device is not shared. It is also a statement of personal style, and is likely to be selected with an eye for fashion, physically customized with stickers or Hello Kitty, and customized in sound, standby screen, and software.
• Always on, always connected. Instead of being turned on only for use, mobile phones are turned off only to preclude interruption for various temporary reasons. This means that users will be reachable in the bathroom. Some cultures consider that turning off or not answering a phone is rude. This allows significant opportunity for ongoing event-driven user interaction.
• Battery powered, since users wander beyond the length of a power cord.
• Wirelessly connected, since users wander. Wireless networks are by their nature inconsistent in coverage. Dead spots, cell overloads, and tunnels all affect the ability to access the network.

These characteristics should remain true as devices evolve.

The Carry Principle applies most directly to communications and control devices, because the desire to stay in touch drives users to leave their personal communications devices on. However, many mobile devices, particularly the mobile general-purpose work device and the mobile general-purpose entertainment device, are likely to be carried for several hours a day. Thus many of the issues and constraints arising from The Carry Principle apply to other devices.