Answer to Question 1
a. We generally don't have the resources necessary to sample randomly. It would take considerable time, money, and energy to collect information from diverse groups of people. So we take the easier route most of the time, using college students. It is an imperfect strategy but it does yield useful results.
b. We may not be able to identify our population of interest specifically enough to be able to specify the population, so we cannot develop a sampling strategy that would generate a probability sample.
c. When psychologists engage in theoretical research, the population may not be of great interest. The test of a theory should produce the expected results, regardless of the nature of either the sample or the population.
Answer to Question 2
People, including psychologists, use the terms culture, race, and ethnicity very imprecisely. One researcher might refer to a person's culture but a second researcher might refer to race or ethnicity in explaining the same behaviors; the researchers might mean the same thing, but they use different terms to refer to the same thing. Or they might really mean different things when they use the terms differently. Unfortunately, there is no consistent agreement about what the terms mean. In addition, the concept of race is problematic because there are not characteristics that we can use to reliably categorize people racially; there will always be confusion because some people resist easy categorization. Many scientists regard race as a social construction rather than as a biological fact.
It is important to distinguish among them because they can refer to useful construct. For instance, ethnicity is often associated with affiliation (e.g., who do you think you belong with), whereas culture pertains to the objects we use (physical culture) or to behaviors (subjective culture).
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