As medical aims, diagnosis, prognosis, intervention and prevention benefit from scientific method, although they have specific characteristics requiring distinct types of design and statistical analysis. The present article aims to provide definitions of the common and differential features of checklists for the main study designs. Distinctions are made between statistical inference and decision, systematic and random error, confirmation and exploration, prediction and intervention, observation and experimentation, and random allocation and random selection. In addition, the main tools available to researchers to control random and systematic errors are described. How all of these elements are contained in the main types of design is discussed. Importantly, checklists are not a measure of the quality of a study but rather represent minimum requirements that aim to improve the quality of scientific reports.