Interesting research topics unique to aging research include comparing the ''maximum lifespan'' for different populations or the examination of the effects of different treatments on lifespan. As mentioned previously, it is generally impractical or impossible to collect data from all of the individuals in the population of interest. As such, we collect data on samples taken from the population and attempt to generalize our sample-specific findings to the larger population. As a result, one can only observe the maximum lifespan of a sample which is sensitive to sample size (David and Nagaraja, 2003). Therefore, in practice, the phrase ''maximum lifespan'' is used to refer to upper percentiles, for example, the 90th percentile (Speakman et al., 2002). Seeking and evaluating interventions that can extend maximum lifespan is an exciting and active field in aging research. For instance, evidence that caloric restriction (CR) increases both the maximum and mean lifespan of many species has been accumulating (Weindruch and Walford, 1988). In this context, the null hypothesis is that there is no difference in maximum lifespan between two independent groups, say one experiencing CR and one not. What is a little surprising is that many studies, including some recent, well-known studies (Hochschild, 1973; Flurkey et al., 2001; and Anisimov et al., 1998) did not conduct any formal statistical hypothesis testing to compare maximum lifespan between treatment groups. Moreover, there is no statistical test accepted as being the ''best'' for testing the difference in lifespan.
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