Epidemiological studies are done to find cause of disease. The factor must be necessary and sufficient. Additional factors required to produce disease are environmental factors and host factors. Types of association are—causal association and non-causal association. Non causal association could be due to biased methods of study or confounding factors. Causality is assessed by measuring- strength of association, consistency of association, specificity of association, time sequence, biological credibility, experimental evidence, dose response relationship.