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A pharmaceutical company tests a new anxiety medication at an urban medical center. Participants are volunteers who called after seeing an advertisement. The sample is 85% college-educated, 75% earn over 80,000annually,and8080,000 annually, and 80% are employed full-time. National census data shows the adult population is 35% college-educated, 45% earn over 80,000, and 60% employed. When researchers generalize results to the entire U.S. population, which concern is most significant?