In the process of evidence-based dentistry the value of each article located by means of a search action has to be determined. This is done based on the research design used. The internal validity of these designs indicates the degree to which alternative explanations might be responsible for the found results. On the basis of internal validity, research designs can be placed on the so-called ladder of ‘evidence’. Clinical experimental studies (randomised controlled trials, cross-over design and split-mouth design), studies in which the researcher introduces changes or intervenes and keeps the other factors constant, have the highest level of internal validity. Observational studies (cohort study, case-control study, cross-sectional study, patient series and case study), in which groups are described and compared in their natural habit, can introduce bias, leading to a lower level of evidence. In order to assess the value of research, it is necessary to have some understanding of what these research designs are concerned with.