Hoer & Attin (2004) also showed that the use of impedance ratio electronic devices did not result in precise determination of the apical constriction, rather, under clinical conditions it was only possible to determine the region between the minor and major apical foramen. In their study, accurate determination of the apical constriction was only successful in 51–64.3% of canals, although the probability of determining the area between minor and major foramen was between 81 and 82.4% of cases. However, Shabahang et al. (1996) showed that when a potential error of ±0.5 mm from the ‘foramen’ was accepted as a clinically tolerable range, the Root ZX was able to locate the ‘foramen’ in 96.2% of cases.