Relative survival: a useful tool to assess generalisability in longitudinal studies of health in older persons

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.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.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.Generalisability (external validity) is the extent to which the results of a study can be applied to other populations. The many threats to the external validity of a study's results include choice of sampling frame, representativeness of the initial sample, and attrition. These issues were discussed in a previous paper [1], and reporting methods were proposed that would enable the reader to assess - at least qualitatively - the generalisability of results from a cohort or longitudinal study. These reporting methods have since been taken further with the publication of the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) initiative [2].A common method of assessing generalisability is to compare demographics, health characteristics, and health service variables between a study sample and population of interest at baseline. Over time, this comparison should be repeated to see if biases are changing. This process relies on data from people who enrol and remain in the cohor