The application of lethal agents of any kind in entomological research is usually aimed at the determination of mortality as related to such factors as dosage, temperature, and time. Mortality is the yard-stick and survivors merely the fraction that did not die. After an arbitrarily selected interval the experimental material is categorized as dead or alive, with sometimes an intermediate group such as “subnormal” inserted. This third class is often essential, for its members do not properly qualify as dead or alive, terms that imply totality. It is also a nuisance, for several reasons: (1) it blends gradually into the other two categories so that classification itself is subject to errors of judgment; (2) its numerical uncertainty affects the other categories not only at the time of initial classification but also later by the death or total recovery of some of its members; (3) as long as any treatment-effect persists, the situation remains quantitatively incomplete.