Focus groups are used by marketers to find out how and why to sell a product. People are randomly selected, who meet certain criteria, to evaluate a product and are then compensated for their opinion. Despite the best laid plans of marketing firms, more than a third of the people in most focus groups in which I've participated have been actors pretending to be a member of the target class. The actors do their best to tell the moderator what they think the people paying the bill want to hear.

The crucial question is whether the presence of actors pretending to be the target class, rather than actual members of the target class, in the marketing focus group make the results invalid? I doubt it. The actors who do this type of thing have been doing so for a long time.

They are familiar with the language of...