Through a combination of descriptive text and exercises, this study presents a common framework for the planning and critical evaluation of epidemiological studies. It describes the principal sources of error in study design, their impact on the results, and the research strategies used to prevent or control different kinds of errors. These strategies are the building blocks used in the design of studies with the overall objective of obtaining accurate results with limited resources. They are illustrated throughout by examples and exercises. The work aims to provide the reader with a clear understanding of the research strategies used in experimental and non-experimental studies.