Final answer:
Researchers infer 1960s drug use from surveys in the 1970s providing retrospective estimates.
Step-by-step explanation:
Researchers draw fairly accurate inferences about drug use during the 1960s by using retrospective estimates based on surveys conducted in the 1970s. Surveys from subsequent decades can provide insight into previous periods through questions that ask about behavior in earlier years. For instance, the National Survey on Drug Use and Health, along with other sociological studies, often use secondary data to analyze changes in society's behaviors and attitudes towards drugs over time.