Recruiting for usability test is hard. (I've said this before.) And it's the most important thing to get right in a test. So how do you decide who to recruit?
Demographics don't describe behavior
If you buy the argument of your marketing department, you will look at the demographics of the various segments and try to match their proportions. You'll know the ages, incomes, educations, ethnicity, and genders of your participants. But does knowing this help you predict behavior or performance? More importantly, with a sample of, say, eight participants, can you generalize discovered usability problems to the broader cohort?
Probably not. Here's an example of why.
Though most video gamers are male, some are female. The problems and successes they have in using a game are similar. And there will be differences within the genders, too. Though most video gamers are young, there are a lot that aren't. The problems they have in using a game are not likely due to differences in age if the participants have similar expertise on the platform and with the game (or similar games).
Behavior describes performance
Instead, the differences in behavior (interaction between the person and the technology) and performance (whether the human is successful in completing technology-mediated tasks) are much more likely to stem from differences in expertise.
Being younger or older doesn't make you an expert at anything necessarily. Having a higher or lower household income doesn't, either. You could argue that education level might, but it usually doesn't unless there's something in the test that is related to a particular domain that the educated person was specifically trained for.
You want people to be motivated to do the tasks you want them to do when they get into your test situation. This is a place where it might make it easier or more difficult to find people. For example, if you want to test an online banking service or find out if someone might sign up for a brokerage account online, it's more likely that the participants will fall into a "mature" category on the age scale than at the younger end or the very old end. And that is just because people in the mature range are more likely to have or want a mortgage than someone who is younger and isn't in the market to buy a house or someone who is older who really would rather have a reverse mortgage. But you might find some on either end, too. But you want to see a range of people with different aptitudes and skill levels.
How do you recruit, then?
Minimize the demographics for small tests, focus on knowledge and proficiency
Skip the demographic questionnaire (or minimize it at least) and focus on what participants have done related to what you're testing. If you are doing a test of a Web site, you might care about what kinds of things do participants do on the Internet and how often they do it. Also, when was the last time? For example, what's the last thing they bought online? Purchasing at an e-commerce site, no matter how well designed the site is, involves complex interaction. It might be a reasonable proxy for searching, narrowing a search, going through a decision process, filling in online forms, handling error and information messages, understanding where in an online process they are, and so on. But it doesn't matter how old participants are, how educated they are, or (usually) what their household income is.
If you're testing how well text messaging works, you want to know whether people do it already and how much. If they don't do texting, you might want some people in your study who have received messages but don't send them. By asking what their recent experiences were related to what you want to test (without giving away your tasks), you can find out about motivation as well as expertise.
And this brings us to a discussion about "novice" versus "expert." But that's another post.