"Practice guidelines are "specific practice recommendations that are based on a methodologically rigorous review of the best evidence on a specific topic. They have the potential to improve the quality of care, the process of care, and patient outcomes."

Grimshaw, J.M. and Russell I. (1993). Effect of clinical guidelines on medical practice: a systematic review of rigorous evaluations. Lancet, 342, 1317-1322.

The best guidelines are based on rigorous scientific evidence (i.e. systematic reviews or randomized controlled trials) – however there are guidelines that can assist in decision making that are based on a consensus of expert opinion.