Supplement ratings are everywhere: stars, scores, badges and league tables, all promising to tell you which product is best. But a rating is only as good as the method behind it, and most ratings never explain their method at all. Before trusting a score, it is worth asking what the score is actually measuring.

The weakest ratings measure popularity. A product with many five-star reviews may simply be a product with many buyers, an aggressive review-request campaign, or a generous discount for reviewers. Popularity can reflect quality, but it can also reflect marketing spend, and the two are impossible to separate from a star average alone. Look for whether the rating distinguishes verified purchases from unverified ones, and whether it discloses how reviews were collected.

Stronger ratings measure the label. They check whether the ingredient amounts are fully disclosed, whether the doses match the research, and whether the product has been tested by an independent laboratory.…