Which is how lots of research, including medical/pharmaceutical [1] studies, are done. [1] Sure, you have to test your new cancer drug on actual cancer patient

Which is how lots of research, including medical/pharmaceutical [1] studies, are done. [1] Sure, you have to test your new cancer drug on actual cancer patients, but why include the 65 year old obese patient, when you could instead include the 32 year old triathlon athlete into you cohort?