Researchers tend to look at many different traits to determine who fits their current study; this could include age, geographic location, gender, interests, products you’ve recently bought, how big your family is, ethnicity, previous health conditions, religious beliefs, etc. depending on their needs.
We want to be clear that if you were ever screened out due to religious beliefs, age, gender, or ethnicity it is never for discriminatory reasons. For example, the researcher needed to hear from people who eat out at least once a week at a specific restaurant. You eat out once a week but you don’t visit the restaurant they were looking for, so they cannot use you for the survey.