Correct Expansion of 'ED50' Values in Clinical and Laboratory Studies

To the Editor.—  In the June issue of the Archives, two brief reports from this department appeared in the LETTERS TO THE EDITOR^1,2 and involved data originally presented as "ED₅₀" values for neuroleptic effects in clinical and laboratory studies. In both letters, the term was changed by a copy editor to verbal expressions indicating population responses or a dose producing an effect in half of the subjects. In fact, both reports involved data concerning doses associated with attainment of a "half-maximum" effect. In the clinical report, this dose corresponded to a degree of sparing of relapse of psychosis halfway between the relapse rate on placebo and the theoretical maximum of 100% protection on active medication. The laboratory data referred to inhibition to half of a maximum behavioral effect induced by a fixed dose of a dopamine agonist. While a traditional definition of "ED₅₀" arose from the median