Walmart marketers have determined in their research design that they want to hear from grocery shoppers in major cities in the United States. They decide to focus on the five metro areas where Kroger has seen the biggest growth: Phoenix, Detroit, Los Angeles, Atlanta, and Dallas$$Fort Worth. Every person $23$ years of age or older in those cities will have an equal chance of being selected to participate in the survey. This is an example of $\_\_\_\_\_\_\_.$