The following four common situation scenarios involve selecting a sample and understanding how a sample relates to a population.
a. A friend interviews every 10th shopper who passes by her as she stands outside one entrance of a major department store in a shopping mall. What type of sample is she selecting? How might you define the population from which she is selecting the sample?
b. A political polling firm samples 50 potential voters from a list of registered voters in each county in a state to interview for an upcoming election. What type of sample is this? Do you have enough information to tell?
c. Another political polling firm in the same state selects potential voters from the same list of registered voters with a very different method. First, they alphabetize the list of last names, then pick the first 20 names that begin with an A, the first 20 that begin with a B, and so on until Z (the sample size is thus 20 x 26, or 520). Is this a probability sample?
d. A social scientist gathers a carefully chosen group of 20 people whom she has selected to represent a broad cross-section of the population in New York City. She interviews them in depth for a study she is doing on race relations in the city. Is this a probability sample? What type of sample has she chosen?