Psychiatric studies are characterised by large numbers of variables, clinical outcomes that are difficult to measure, small sample sizes and conflicting results. Study designs are classified along the axes of time (longitudinal or cross-sectional design), level of causal inference (descriptive or analytical design) and role of the investigator (observational or experimental). Descriptive designs can be used to examine associations between exposures and outcomes in such a way that the results may lead to the formulation of more specific hypotheses regarding the causal implications of the exposure-disease relationship. The case-control design is suitable to examine aetiological heterogeneity or different exposures resulting in the same outcome, because a range of exposures in patients and control subjects can be assessed. Medical scientific studies generally have two main interests in the examination of the results: examination of the morbidity force of a disease phenotype in populations and examination of associations.