Ideally, the results of such a study should be based on randomized, controlled clinical trials. Because of the small number of these, investigators had to compromise and chose to include prospective and retrospective clinical studies and to compare their results versus a constant. This constant, which was defined as zero, assumes that no significant tooth movement would take place in untreated individuals during a short period of less than a year, and that the standard deviation is the error of the method. If randomized, controlled trials cannot be carried out because of the nature of the treatment provided with these appliances, investigators consider that the error of the method based on double recordings could be a valuable element in the evaluation of relevant publications.