"a researcher who examines the relationship between individuals' tooth flossing frequency and the incidence of heart disease in these individuals is using the research design."

"Correlation"

Explanation:

According to the information given to us in the statement we can see that the researcher is using a "Correlation Research Design" when making his investigation. This specific technique of research uses two different variable and investigates the relationship between them as well as the similarities and differences they have, without any outside variables interfering with the investigation.

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