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Endodontic literature is replete with surgical success-failure studies. Efforts to compare them should be avoided because there are so many variables within each of these studies. Among these variables can be surgical technique, clinical ability, restorative materials, the use of pooled data, and interpretation of data. Many of these studies serve as an historical reference from which current-day evaluators can build and design studies that are more contemporary and relevant to modern endodontic surgery. If a study is to be considered valid, it should strive to examine a well-controlled population and use a reproducible and rigorous surgical protocol. It is often difficult for the reader to know how much credibility to place on a given study until he or she understands the criteria for the various types of success-failure studies.