Testing and analytical procedures for laboratory studies involving nonresponders during a limited observation period - An illustration using male sexual behavior in rats

In many laboratory studies, a subpopulation of subjects fails to exhibit th e response under investigation during the period of observation. For exampl e, within any population of male rats, there is significant variation in th e expression of sexual behavior in the presence of a receptive female. Some males may never display the full sequence of behaviors leading to ejaculat ion within the typical time frame of the testing session, with the resultin g lack of behavioral response presenting problems in the analysis of the da ta. Conventional strategies range from screening such males from the study or dropping them from the analysis to constructing new variables based on e stimates from existing parameters or increasing the length of the test sess ion to capture sexual responses in a greater portion of males. Herein, we p resent an alternative strategy for analyzing data where outcomes are absent due to the limited observation period. Survival regression analysis enable s inclusion of all subjects in the analysis whether or not they have shown the behavior of interest. Use of such a strategy not only has potential to reveal new results but also guards against bias from excluding nonresponder s from the study or dropping more males from one experimental condition tha n another. Furthermore, this procedure can be helpful in generating the con ditional probability (increase, decrease, or constant) of the response with the passage of time based on the hazard function and in estimating paramet ers for establishing an optimal behavioral test length for future studies. (C) 2001 Elsevier Science Inc. All rights reserved.