Anticoagulant therapy with coumarin drugs requires tests on the patient's blood to determine the dose rate for the drug. The tests involve measuring the effect of a preparation of a certain reagent, thromboplastin, in accelerating the clotting of the patient's blood. Features of the World Health Organization's recommendation on the appropriate laboratory procedure for a calibration assay and evaluation of results call for comment. The standard method involves estimation of a linear structural relation, a statistical technique long known but seldom finding practical application. This paper reviews its history, as a prelude to discussing strange mistakes in earlier publications; it also examines assessment of the precision of estimation and other aspects of the statistical method.