title: Relative survival: a useful tool to assess generalisability in longitudinal studies of health in older persons.
abstract: BACKGROUND
  Generalisability of longitudinal studies is threatened by issues such as choice of sampling frame, representativeness of the initial sample, and attrition. To determine representativeness, cohorts are often compared with the population of interest at baseline on demographic and health characteristics. This study illustrates the use of relative survival as a tool for assessing generalisability of results from a cohort of older people among whom death is a potential threat to generalisability.
  
  
  METHODS
  The authors used data from the 1921-26 cohort (n = 12,416, aged 70-75 in 1996) of the Australian Longitudinal Study on Women's Health (ALSWH). Vital status was determined by linkage to the National Death Index, and expected deaths were derived using Australian life tables. Relative survival was estimated using observed survival in the cohort divided by expected survival among women of the same age and State or Territory.
  
  
  RESULTS
  Overall, the ALSWH women showed relative survival 9.5% above the general population. Within States and Territories, the relative survival advantage varied from 6% to 23%. The interval-specific relative survival remained relatively constant over the 12 years (1996-2008) under review, indicating that the survival advantage of the cohort has not diminished over time.
  
  
  CONCLUSIONS
  This study demonstrates that relative survival can be a useful measure of generalisability in a longitudinal study of the health of the general population, particularly when participants are older.
authors.affiliation: School of Population Health, University of Queensland, Herston QLD 4006, Australia. r.hockey@sph.uq.edu.au.
authors.forename: Richard
authors.initials: R
authors.lastname: Hockey
authors.forename: Leigh
authors.initials: L
authors.lastname: Tooth
authors.forename: Annette
authors.initials: A
authors.lastname: Dobson