Standardised comparisons of the prevalence of asthma are important for generating and testing hypotheses about the causes of asthma and, in particular, the causes of the global increases in its prevalence. Such surveys may involve international comparisons1 ,2 or comparisons between “subpopulations”—for example, age, sex, socioeconomic, or regional subgroups—within a single geographical population or country. It is important that such population surveys should use the most practical and valid methods for measuring the prevalence of asthma in populations and differences between populations. This is inherently problematic because of the difficulties in defining asthma and the practical considerations that must be taken into account in achieving high response rates in large random population surveys.

Furthermore, there is no single test or pathognomonic feature which defines the presence or absence of asthma. The variability of the condition also means that evidence of it may or may not be present on the day or at the time of assessment. Thus, a diagnosis of asthma in clinical practice is made on the basis of combined information from history, physical examination, and respiratory function tests, often over a period of time. However, comparisons of the prevalence of diagnosed asthma between populations are fraught with difficulty, as the differences in diagnostic practice may be as great in magnitude as the real differences in asthma morbidity. It may be possible to address some of these issues by adopting common criteria for asthma diagnosis and applying these uniformly in prevalence studies. However, for large scale prevalence studies this is not practical because of the need for repeated contacts between study participants and doctors. Thus, surveys comparing the prevalence of asthma between populations usually focus on self-reported (or parental reported) “asthma symptoms” rather than diagnosed asthma.1 ,2

An alternative approach to symptom questionnaires has been …