Analysis of censored data in multi-factorial analgesic test

Animal pain testing is essential for the development of new analgesic drugs, where appropriate data analyses as well as appropriate multi-factorial design of experiments are necessary to obtain meaningful results in an efficient fashion. The tail withdrawal experiment is one of the pain tests in which a rhesus monkey is restrained in a chair from which its tail hangs free by so it can be immersed in warm water. The monkeys consistently kept their tails in 38-40 degrees C water for an extended period of time, and thus, the data were censored at 120 sec. The effect of temperature on the tail withdrawal latency was evaluated using three monkeys with a randomized block design. The effect of morphine on the thermal sensitivity was also evaluated. A Friedman-type two-way analysis of variance (Mack-Skillings test) demonstrated that the effects of both temperature and the animals were significant. The effect of repeated measurement in one animal was not significant using the Friedman test, indicating that the significance of the effect of animals could be attributed to the difference in the intrinsic thermal sensitivity between animals. This method, together with a graphical approach, may prove to be valuable for assessing the sensitivity and reproducibility of an experimental condition, as well as the pharmacological effects of analgesic drugs.