EXAMPLE 3.6 Interviewing Customers at the Mall

Manufacturers and advertising agencies often use interviews at shopping malls to gather information about the habits of consumers and the effectiveness of ads. A sample of mall customers is fast and cheap. But people contacted at shopping malls are not representative of the entire U.S. population. They are richer, for example, and more likely to be teenagers or retired. Moreover, mall interviewers tend to select neat, safe-looking subjects from the stream of customers. Decisions based on mall interviews may not reflect the preferences of all consumers.