Epidemiological studies sampling problems

All human studies dealing with chronic mustard disease processes are retrospective and fraught with the problems inherent in retrospective studies. These problems include bias in the sampling populations lack of epidemiological controls for the effects of smoking, lifestyle, race, gender, age, or exposure to other chemicals differential quality of available health care and incorrect diagnosis.6 These limitations make absolute interpretation of the studies difficult. 

On occasion, such a survey may indicate the need for a more formal epidemiological study. Skin complaints may, for example, be widespread but unusually hard to explain. Such investigations should never be undertaken by clinicians without previous epidemiological and statistical consultation. Coenraads and Nater (1987) have published a useful introduction to the problems that may arise, including true prevalence estimation, bias, confounding variables and sample size. Questions of disease definition and inter-observer variability are not necessarily familiar to clinicians, who may therefore need to seek epidemiological advice at the earliest opportunity. Questionnaires are frequently designed that ask for far more detail than can possibly be usefully analysed statistically, and they should always be piloted first in order to achieve validity. 

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