Abstract
BackgroundGeneralisability 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.MethodsThe 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.ResultsOverall, 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.ConclusionThis 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.
Highlights
Generalisability of longitudinal studies is threatened by issues such as choice of sampling frame, representativeness of the initial sample, and attrition
In 1996, more than 40000 women responded to the initial survey; they were reasonably representative of the general population of Australian women in each age group, compared with data from the 1996 Australian Census there was over-representation of women who were born in Australia, employed and had a university education [4]
Relative survival There were 3661 deaths (29.4%) amongst the 12 432 women born in 1921-26 who participated in the baseline survey of the Australian Longitudinal Study on Women’s Health (ALSWH)
Summary
Generalisability of longitudinal studies is threatened by issues such as choice of sampling frame, representativeness of the initial sample, and attrition. 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. 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. Repeated assessments of generalisability are appropriate in longitudinal cohort studies where drop out is potentially not a random phenomenon (e.g., related to characteristics of people who cease to participate)
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