title: Testing for differences in distribution tails to test for differences in 'maximum' lifespan.
abstract: BACKGROUND
  Investigators are actively testing interventions intended to increase lifespan and wish to test whether the interventions increase maximum lifespan. Based on the fact that one cannot be assured of observing population maximum lifespans in finite samples, in previous work, we constructed and validated several tests of difference in the upper parts of lifespan distributions between a treatment group and a control group by testing whether the probabilities that observations are above some threshold defining 'old' or being in the tail of the survival distribution are equal in the two groups. However, a limitation of these tests is that they do not consider how much above the threshold any particular observation is.
  
  
  METHODS
  In this article we propose new methods which improve upon our previous tests by considering not only whether an observation is above some threshold, but also the magnitudes by which observations exceed the threshold.
  
  
  RESULTS
  Simulations show that the new methods control type I error rates quite well and that the power of the new methods is usually higher than that of the tests we previously proposed. In illustrative analyses of two real datasets involving rodents, when setting the threshold equal to 110 (100) weeks for the first (second) datasets, the new methods detected differences in 'maximum lifespan' between groups at nominal alpha levels of 0.01 (0.05) for the first (second) datasets and provided more significant results than competitor tests.
  
  
  CONCLUSIONS
  The new methods not only have good performance in controlling the type I error rates but also improve the power compared with the tests we previously proposed.
authors.affiliation: Department of Biostatistics, Section on Statistical Genetics, University of Alabama at Birmingham, Birmingham, Alabama, USA. guiming@uab.edu
authors.forename: Guimin
authors.initials: G
authors.lastname: Gao
authors.forename: Wen
authors.initials: W
authors.lastname: Wan
authors.forename: Sijian
authors.initials: S
authors.lastname: Zhang
authors.forename: David T
authors.initials: DT
authors.lastname: Redden
authors.forename: David B
authors.initials: DB
authors.lastname: Allison
mesh_terms: D003627:Data Interpretation, Statistical; D006801:Humans; D008136:Longevity; D015233:Models, Statistical; D019370:Observation / Q000379:methods*; D012107:Research Design