Knowing how effective vitamins & herbal items are
I've been studying nutrition and prevenitive medicine for years, but outside of reading the study itself, it's hard to find out how effective something is. Somehow I've never seen the lookup info in the Mayo Clinic's website. Keep in mind the A-F scale doesn't imply it's effectiveness, but the amount or reliability of the studies that have proven a certain problem. Things that are unknown but not disproven are rated C, which you'll find is about 80% of ailments. I'd assume C is pretty good, because there is evidence that proves it can be effective for many people and it hasn't been disproven. If you come across B, it's pretty darn effective and if you see an A, well consider it a miracle cure (well almost).
http://www.mayoclinic.com/health/coenzyme-q10/NS_patient-coenzymeq10