General principles of epidemiology are reviewed, using as an example the chronic non-specific respiratory diseases in Canada. Lack of agreement in definition and classification seriously hampers the use of governmental statistics to study the epidemiology of these diseases. Though Canada is experiencing a mid-century epidemic of deaths from emphysema, evidence is presented which suggests that provincial variation in this death rate is artificial and is likely apparent because physicians do not categorize deaths from this and related diseases in any similar fashion. Three-year-average age-standardized death rates by provincial areas for emphysema with or without bronchitis, asthma, chronic bronchitis and bronchiectasis are presented in evidence that provincial variation is largely obliterated when all chronic non-specific respiratory diseases are considered as a single nosological unit. Advantages and disadvantages of a recent clinical classification of these diseases are reviewed, and it is concluded that such a classification has a great deal to offer epidemiologists or clinicians studying the cause and natural history of these diseases.
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