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63 On-board and intercept surveysâself-administered surveys distributed on board buses and rail cars and in stations, and personal interviews conducted in these environments. Sampleâgroup of people selected to be surveyed and that the researcher attempts to contact. Sample frameâlisting of the study population from which the sample will be drawn. Sampling errorâdifference between the true (but unknown) value and observed values if, hypothetically, the survey were repeated numerous times. Simple random samplesâsample in which each person in the study population has an equal chance of being selected. Stratified sampleâsample divided into separate subsamples, each of which is then sampled as a simple random sample. Study populationâpopulation to which the researcher can gain access. Theoretical populationâpopulation to which researchers wish to generalize. GLOSSARY