Epidemiology Study Types and Equations

Randomized Control Trial (RCT)

Experimental study following disease-free people; exposure and disease have not yet occurred.

Cohort Study

Observational study following disease-free people prospectively or retrospectively.

Case-Control Study

Observational study comparing previous exposure frequency between identified cases and controls.

Cross-Sectional Study

Observational study taking a snapshot of exposure and disease at one point in time.

Ecological Study

Observational study where timing varies.

Prevalence Ratio (PR)

Prevalence in exposed divided by prevalence in non-exposed.

Prevalence Difference

Prevalence in exposed minus prevalence in non-exposed.

Prevalence Equation

Cases present at a specified time divided by population at that time.

Case Fatality Equation

Individuals dying from a specific disease divided by individuals with the disease.

Proportionate Mortality Equation

Deaths caused by a specific disease divided by total number of deaths.

Cumulative Incidence (Risk) Equation

New cases during a period divided by persons at risk during that period.

Incidence Density (Rate) Equation

New cases during a period divided by total person-time at risk.

Mortality Rate Equation

Total deaths from all causes in one year divided by population at midyear.

Cause-Specific Mortality Rate Equation

Deaths from a specific cause in one year divided by population at midyear.