IF in a biological assay the mean responses, y11, y12, of two groups of n animals to doses of a standard preparation the logarithms of which are x11, x12, are compared with the mean responses, y21, y22 to doses of the preparation under test the logarithms of which are x21, x22, it is often possible to express the relationship between dosage and response adequately by the equations: Y1 = a + bx, Y2 = a + b(x - M), (1)