Cost-Justifying Usability, 2nd Edition

perhaps 5% instead of 20% (which was the basis for using 5% in the calculation

in Table 13.4).

Overlap of Populations

Even after addressing the needs of a given disability, such as visual impairments,

some people are likely to have other disabilities or barriers, such as language

difficulty or lack of computer experience. To compute the number of people

affected when you address more than one barrier at a time, you need to figure

out how much the populations overlap. If 3.2% of people have difficulty using

their hands and 44% have a screen size of 800 ¥ 600, you don’t get an additional

47.2% benefit from addressing both. You have to subtract the people ...