Error of the method: what is it for?

The aim of this paper is to demonstrate the importance of evaluating the error of the method in Orthodontic scientific studies. Special emphasis will be given to the scientific importance and the different types of the error of the method (systematic and casual), the statistical tests most commonly used to quantify these errors and the clinical meaning of the error of the method for the interpretation of results obtained from orthodontic treatment. To conduct studies within the Dentistry field, particularly in Orthodontics, the researcher needs to measure continuous quantitative variables, i.e., angles or millimetric distances (for instance, the cephalometric variables of lateral cephalograms and/or variables of study casts). To analyze a lateral cephalogram, cephalometric points are traced and distances and angles are measured. However, it is known that these values will vary between examiners and for the same examiner at different moments. It is difficult to obtain always the same exact value. Thus, most studies do not provide accurate measurements with 100% reliability. Because the variability of measurements, that is, the " errors " will always exist, the magnitude and source of errors are more important than the errors themselves. 2 In scientific research, measurement without the information on the magnitude of the error makes no sense. Therefore, it is extremely important not only to determine the examiner's accuracy in obtaining measurements, but also to estimate all possible sources of error in a research. A recently published study identified the 100 most cited articles in the orthodontic literature between 1975 and 2011. 4 At the top of that list, with 545 citations, is the study conducted by Houston, 3 which aimed at performing error analysis for the measurements carried out in orthodontic research. This fact highlights the importance of the error of the method for orthodontic research. The main source of measurement error in Orthodon-tics is the interference of the examiner in the measurement process. While the examiner does not interfere in the process of measuring a person's weight (because it is enough just to put the individual onto a scale), cephalometry depends on his knowledge and ability to locate the cephalo-metric points. Ideally, the methods employed should be accurate enough so as to allow their reproduction. When this accuracy is somehow compromised, errors occur. They can be defined as the difference between the value obtained during the process of measurement and the real value of the magnitude of measurement. …