Answer: Unobtrusive measure
Explanation:
Unobtrusive measurement is a measurement that deals with getting results from mere ideas relating or surrounding a particular subject or survey without disturbing or involving the party or subjects related to the study.
Here, when a researcher indirectly measures the frequency at which a supermarket purchases a particular health drink and based his assumptions on the idea that the most popular health drinks are the ones more frequently bought then the researcher is using an Unobtrusive measure.