The aim of this study was to investigate the possible relationship between interfacial forces, duration of instrumentation and effectiveness of root canal shaping by filing. All factors were standardised except the manner of manipulation of the files and the dependent variables (interfacial force and duration of filing). The outcome measured was the final prepared canal shape. The study tested the hypothesis that if interfacial forces and duration of filing had predictive value for effective canal shaping, a relationship between them should be apparent. If it was not, it could be inferred that the remaining uncontrollable variable, "manner of file manipulation" had a dominating influence. 18 operators used a standard filing technique to instrument a root canal each in single rooted teeth matched for anatomy. The teeth were mounted in a custom-made transducer designed to measure laterally applied interfacial forces between file and dentine. The length of time taken with each instrument in the series was measured giving a total duration of instrumentation. The final canal shape measured by standardised "before" and "after" radiographic images of the roots was judged against an "ideal" canal shape calculated from the size of instruments used and based on the preliminary canal shape. Although some trends were revealed, no obvious relationship was demonstrated between interfacial force, duration of filing and final canal shape. Operators clearly used a range of interfacial forces characteristic for each individual and for the different file sizes. Both light and heavy forces could produce a satisfactory canal shape. Use of light or heavy forces did not obviously affect speed of preparation. Tactile discrimination was not dependent on use of light or heavy forces. It was inferred that the "manner of file manipulation" remained undefined and was probably the dominant factor influencing outcome of canal shaping.