Tissue response partial agonist efficacy from

Attempting to Estimate the Efficacy of a Partial Agonist from the End Response of a Complex Tissue... 

The partial agonist concept was additionally refined by Stephenson (33), who introduced the concept of efficacy, e, which differed from intrinsic activity in that the latter was defined as a proportion of the maximal response [effect = a (RL)]. This concept was extended to situations where a maximal response to an agonist could occur when only a small proportion of the total number of receptors on a tissue were occupied, as in the situa-... 

---