An activity will be pursued only if its benefits are equal to or greater than the costs.

From a design perspective, the cost-benefit principle is typically used to assess the financial return associated with new features and elements. The cost-benefit principle can also be applied to determine design quality from a user perspective.
If the costs associated with interacting with a design outweigh the benefits, the design is poor. If the benefits outweigh the costs, the design is good. For example, walking some distance to see a museum exhibit constitutes a cost. The level of interest in the exhibit constitutes a benefit. Thus, if the level of interest outweighs the cost of the walk, the exhibit design is good.

The quality of every design aspect can be measured using the cost-benefit principle. How much reading is too much to get the point of a message? How many steps are too many to set the time and date of a video recorder? How long is too long for a person to wait for a Web page to download? The answer to all of these questions is that it depends on the benefits of the interaction. For example, the often-cited maximum acceptable download time for pages on the Internet is ten seconds. However, the acceptability of download time is a function of the benefits provided by the downloaded page. A high-benefit page can more than compensate for the cost of a download taking longer than ten seconds. Conversely, a low-benefit page cannot compensate the cost of any download time. Reducing interaction costs does improve the quality of the design, but to simply design within Cost limits Without Consideration of the interaction benefits misses the point of design altogether-i.e, to provide benefit.

A common mistake regarding application of the cost-benefit principle is to presume which aspects of a system will be perceived as costs, and which will be perceived as benefits. For example, new design features or elements that excite designers are often never used or even noticed by people who interact with the design. In many cases, such features and elements increase the design’s interaction costs by adding complexity to the system. in order to avoid this, observe people interacting with the design or similar designs in the actual target environment. Focus groups and usability tests are valuable in assessing the cost benefits of a design during development, when natural observation is not possible.

Consider the cost-benefit principle in all aspects of design. Do not make design decisions based on cost parameters alone without due consideration of the benefits realised from interactions. Verify cost-benefit perceptions of target populations through careful observations, focus groups, and usability tests.