Think about the last time you bought a shirt. You probably didn’t inspect the stitching or test the fabric. You trusted the store, because the store has a strong incentive to sell good shirts.
Milton Friedman uses this everyday example to make a point about medicine. People who go to the Mayo Clinic aren’t vetting each physician themselves. They trust the clinic, which has every reason to choose able, well-trained doctors.
Friedman’s argument is that without licensure and its restrictions on entry, we would have seen much more of this kind of group practice, and a much more effective way of selecting for quality in medicine.