Table 1.

Observational study design measures of disease, measures of risk, and temporality.

Study design	Measures of disease	Measures of risk	Temporality
Ecological	Prevalence (rough estimate)	Prevalence ratio	Retrospective
Proportional mortality	Proportional mortality Standardized mortality	Proportional mortality ratio Standardized mortality ratio	Retrospective
Case-crossover	None	Odds ratio	Retrospective
Cross-sectional	Point prevalence Period prevalence	Odds ratio Prevalence odds ratio Prevalence ratio Prevalence difference	Retrospective
Case-control	None	Odds ratio	Retrospective
Retrospective and prospective cohort	Point prevalence Period prevalence Incidence	Odds ratio Prevalence odds ratio Prevalence ratio Prevalence difference Attributable risk Incidence rate ratio Relative risk Risk ratio Hazard ratio	Retrospective only Both retrospective and prospective Prospective only