Sample sizes for comparing means of two lifetime distributions with type II censored data: application in an aging intervention study

A Randomized Two Stage Adaptively Censored Design With Application to Testing

A two stage randomized design is developed for two treatment clinical trials in which response variables are exponential and the observations are censored by using failure censoring and time censoring in the first and second stages respectively. The censoring time for the second stage is determined from the outcomes of the first stage. An application to testing for the equality of treatment effects is given along with a comparative study with relevant properties.

A carboxyfullerene SOD mimetic improves cognition and extends the lifespan of mice

In lower organisms, such as Caenorhabditis elegans and Drosophila, many genes identified as key regulators of aging are involved in either detoxification of reactive oxygen species or the cellular response to oxidatively-damaged macromolecules. Transgenic mice have been generated to study these genes in mammalian aging, but have not in general exhibited the expected lifespan extension or beneficial behavioral effects, possibly reflecting compensatory changes during development. We administered a small-molecule synthetic enzyme superoxide dismutase (SOD) mimetic to wild-type (i.e. non-transgenic, non-senescence accelerated) mice starting at middle age. Chronic treatment not only reduced age-associated oxidative stress and mitochondrial radical production, but significantly extended lifespan. Treated mice also exhibited improved performance on the Morris water maze learning and memory task. This is to our knowledge the first demonstration that an administered antioxidant with mitochondrial activity and nervous system penetration not only increases lifespan, but rescues age-related cognitive impairment in mammals. SOD mimetics with such characteristics may provide unique complements to genetic strategies to study the contribution of oxidative processes to nervous system aging.

Effect of a Guarantee Time on Sample Size Determination for Testing the Ratio of Means from Two Lifetime Distributions

This paper studies the problem of sample size determination for testing the ratio of means from two lifetime distributions when thresholds, or guarantee times, are present in the survival distributions. We consider several different approaches of sample size determination based on different distributional assumptions and discuss the role that guarantee times play in these sample size computations. We show that the guarantee times, if they exist, can significantly affect the sample size computations. The Aging Intervention Testing Program from the National Institute on Aging is used to demonstrate our results.