Relative potency is the ratio of the doses of two analgesics required to produce the same analgesic effect.
By convention, the relative potency of each of the commonly used opioids is based upon a comparison with 10 mg of parenteral morphine.

Hanks G and Cherny N. Oxford textbook of palliative medicine. 2nd ed. Oxford. Oxford University Press. 2003. pp. 331-354.