Closer investigation, however, indicated that the prevalence of back symptoms was similar in both regions and that it was patients' consultation habits that varied. Thus, in this instance correlations based on general practitioner consultation rates would be quite misleading. A study based on rates of admission to hospital for perforated peptic ulcer would probably be reliable as in affluent countries almost all cases will reach hospital and be diagnosed. On the other hand, unbiased ascertainment of disorders such as depression or Parkinson's disease may be difficult without a specially designed survey. When there is doubt about the uniformity of ascertainment, it may be necessary to explore the extent of any possible bias in a validation exercise.
Time trends Many diseases show remarkable fluctuations in incidence over time.
Rates of acute infection can vary appreciably over a few days, but epidemics of chronic disorders such as lung cancer and coronary heart disease evolve over decades. If time or secular trends in disease incidence correlate with changes in a community's environment or way of life then the trends may provide important clues to aetiology. Like geographical studies, analysis of secular trends may be biased by differences in the ascertainment of disease. As health services have improved, diagnostic criteria and techniques have changed. Furthermore, whereas in geographical studies the differences are accessible to current inquiry, validating secular changes is more difficult as it depends on observations made and often scantily recorded many years ago.