Notes on the use of historical controls

The principles and methods of incorporating historical controls in four cases (C1, stable control; C2, rare occurrence of responses; C3, small group size; C4, historical control as a reference) are discussed. Two points are emphasized: one is that the historical control should be regarded as a given condition and the other is that the historical control should be used conservatively. Incorporating historical controls is recommended only when it is advantageous under the conditional evaluation of the performance and even in the conservative use of controls. For case C1, adjusting the critical value for the Cochran-Armitage trend test is proposed. For case C2, a modified conditional trend test proposed by Yanagawa et al. is appreciated as a proper procedure. For case C3, a conservative use of interblock information is discussed. The incorporation of the historical control is not recommended for case C4.