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144 PART II: Descriptive MethodsKey ConceptKey ConceptApproaches to Sampling• Two approaches to selecting a survey sample are nonprobability samplingand probability sampling.• Nonprobability sampling (such as convenience sampling) does notguarantee that every element in the population has an equal chance ofbeing included in the sample.• Probability sampling is the method of choice for obtaining a representativesample.• In simple random sampling, each element of the population has an equalchance of being included in the sample; in stratified random sampling, thepopulation is divided into subpopulations (strata), and random samples aredrawn from the strata.There are two basic approaches to sampling—nonprobability sampling andprobability sampling. In nonprobability sampling we have no guarantee thateach element has some chance of being included and no way to estimate theprobability of each element’s being included in the sample. In the computerservicessurvey we described earlier, if a researcher interviewed the first 30 studentswho entered the library, she would be using nonprobability sampling.Clearly, not all students would be equally likely to be at the library at thatparticular time, and some students would have essentially no chance of beingincluded in the sample (e.g., if at work or in class).By contrast, if the researcher were to select 100 students randomly from theregistrar’s list of enrolled students, she would be using probability sampling.In probability sampling, all registered students (elements) have an equalchance of being included in the sample. We can describe this researcher’sapproach as probability sampling because her sampling procedure (i.e., randomselection from a predetermined list) allows all students to have an equalchance of being selected for the survey. Probability sampling is far superior tononprobability sampling in ensuring that selected samples represent the population.Thus, the researcher who selects 30 students randomly from the registrar’s listof students is more likely to have a representative sample than the researcherwho bases her survey results on the first 30 students who show up at thelibrary.Nonprobability Sampling The most common form of nonprobability samplingis convenience sampling. Convenience sampling involves selecting respondentsprimarily on the basis of their availability and willingness to respond. For example,newspapers often publish the comments of “the person on the street.”Their comments may make interesting reading, but their opinions likely do notrepresent those of the wider community. This lack of representativeness arisesbecause convenience sampling is nonprobability sampling, and we can’t be surethat every person in the community had a chance to be included in the sample.Convenience sampling also is involved when people respond to surveysin magazines because the magazine has to be available (and purchased), andpeople must be willing to send in their responses. The “participant pool” thatis tapped by many psychologists at colleges and universities is a convenience