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Teaching method influenced root filling quality?312the possibility that statistically significant differences were left undetected. For example, although the difference in percentage of root fillings of good quality made by students who attended the former programme (47%) and those who attended the revised programme (62%) was substantial (15% difference), this difference was not statistically significant. We, therefore, avoided drawing firm conclusions regarding these data.Determining the quality of a root canal treatment by assessing the quality of root fillings through interpreting periapical radiographs is arguable. Unfortunately, in clinical endodontology, respecting ethical grounds, we do not really have better alternatives. Besides, this method is often used in endodontic research and practice, and there is evidence that quality of root fillings is related to treatment outcome.3,4The intra-observer reliability was substantial and moderate respectively; the interobserver reliability was fair.25 The observer variability in this study was, however, higher than that in similar studies.26,27 Our observer reliability might have been higher if we had determined it per criterion, like Koch et al.26 did in their study for the density of the root filling and for its length separately. Instead we observed on tooth level including several criteria (i.e. length and density of the root filling, as well as occurrence of procedural errors). Although Dahlström et al.27 had a similar approach, they reached a higher intra-observer reliability than we did. They assessed the periapical radiographs jointly and determined the intra-observer reliability of their consensus. Such a consensus might be more consistent than a single person’s observations. We calculated the observer reliability between the single observers and not for the consensus. However, the consensus observations were the dependent variable “quality of root filling” reported and analysed in our study.Three periapical radiographs (pre-operative, length measurement and final) for a root canal treatment are recommended according to the Quality Guidelines for endodontic treatment of the European Society of Endodontology; in some cases they recommend to take a fourth periapical radiograph (master cone) to check the preparation before filling the root canal and they mention that it may be necessary to take more than one length measurement radiograph resulting in a maximum of five radiographs for a root canal treatment.28 In the present study, on average, eight periapical radiographs were taken for a root canal treatment, and three of those radiographs have failed. Although this gives the impression that, on average, five recommended periapical radiographs were available, in fact, in nearly half of the treatments not all the recommended periapical radiographs were existing. In difficult root canal treatments, extra periapical radiographs may be of Annemarie Baaij.indd 31 28-06-2023 12:26