Epidemiological studies are research investigations that focus on the distribution and determinants of health-related states or events in specific populations. These studies help identify risk factors for diseases and evaluate the effectiveness of interventions. They can be observational, where researchers observe subjects without manipulation, or experimental, where they actively intervene. Common types of epidemiological studies include cohort studies, case-control studies, and cross-sectional studies. Each type has its own methodology and purpose, allowing researchers to gather valuable data on how diseases spread and affect different groups, ultimately guiding public health decisions and policies.