3.2.1 Cross-sectional studies
Cross-sectional studies measure the prevalence of conditions or characteristics of people in a population at a point in time or over a short period. Although they are essentially descriptive studies, their results can often suggest causative or risk factors associated with particular illness or behaviour; for instance, the causal relationship between cataracts and vitamin status was originally investigated through a cross-sectional study (Coggon et al., 2003). They may also be used to ascertain the prevalence of a health-related behaviour, such as the wearing of seat belts or participation in exercise. In cross-sectional studies, it is not always necessary to investigate the whole population: a sample is usually sufficient, provided that the individuals in the sample are representative of the total group under consideration. Cross-sectional studies are useful in planning public health interventions.
A population or group can be studied in a variety of ways: by questionnaire, by taking measurements (such as blood pressure), by analysing blood specimens (e.g. for blood cholesterol levels), or by examining healthcare records.