Because lifespan is a continuous variable, it is straightforward to apply a t-test to compare the maximum lifespan between two groups, or to apply ANOVA to compare maximum lifespan among multiple groups. We call this the conditional t-test (CTT) approach. It can be easily generalized to compare means across multiple groups by conditional ANOVA (Goodrick et al., 1990). If the lifespan of the sample is normally distributed, it is apparent that the distribution of the lifespan in the upper percentile has a truncated normal distribution, not a symmetric distribution. We already know that the t-test is a parametric method and that one of its assumptions is that the data are derived randomly from a normally distributed population. Therefore, in this instance, the application of the CTT is questionable. In fact, in a simulation study, Wang et al. (2004) showed that the CTT
does have an inflated type I error rate and is invalid for comparing maximum lifespan.
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