The lessons from the market seem to be that design for purpose can be manifested in the form of a specific intended role, form factor, input style, or software (or indeed any combination of these), depending on whether the market is horizontal, vertical, or consumer.
Buyers concerned with the viability of a vendor or the popularity of a platform can use the fitness-for-purpose test as a powerful guide. At the low end of the device scale, successful platforms tend to be tightly constrained and fitted to an intended purpose, and in general the smaller a device, the more users seem to treat it as a tool that must fit its purpose rather than as a platform to be molded to various tasks.
At the high end--certainly for laptops and above, and probably including tablets--users expect a general-purpose, versatile platform akin to the familiar PC and purpose-specific appliances fail.
One fascinating question that remains to be answered is where the crossover occurs. Somewhere between the successful high-end PDAs of today such as the Hewlett-Packard iPAQ Pocket PC and the smallest, lightest laptops lies a gray area where it is far from clear whether the need to design for purpose or the desire for versatility dominates. Perhaps no device can satisfy both requirements. Until the answer becomes clearer, any new designs venture into that design space at their own risk, and users must proceed with great caution.
Carl Zetie is VP of research at Giga Information Group.
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