Differences between subjects that may influence the effect being studied. For some studies these individual differences can mask the thing being examined (create noise). Ways to reduce the impact of individual differences in such situations include using a very narrow user group, having the same users perform on different systems (a within-subjects design), and choosing participants of the same age and experience to reduce noise. Each of these has implications for the validity of results. Within interaction design and user experience design it is crucial to remember that people are different. Sometimes you have to create a 'one size fits all' application, but if you can understand differences between users this can lead to better designs, whether different applications targeted at different user groups, customisation options within a single application, or intelligent interfaces that adapt themselves to the user.
