The objective of this study was to show that if the double records of each patient are followed at the same time, it is necessary to draw a control chart for the differences between the measured values and to calculate the compliance limits and repeatability coefficients. A default error, assuming the mean differences are zero, called Dahlberg errors in orthodontics and facial orthopedics, can only be calculated to estimate accuracy if accuracy is already demonstrated. When recording two measured values, it is necessary to calculate the conformity limits and repeatability coefficients. A graph with differences in measured values should be presented and outliers discussed. According to ISO 5725-1,[1], the general term “accuracy” is used to describe the proximity of a measurement to the actual value. The chord is perfect only if the points are located along the regression line, that is, a line with the equation Y = X, while the correlation is perfect when the points are located along a straight line. For X-rays, although the correlation coefficient is 0.9951, the slope is significantly different from the 1st Houston5 (1983) indicated that there is a significant difference between X-ray measurements and that standardization is a prerequisite for better results. Houston5 (1983) also calculated the correlation coefficient between intermediate measurements (time 1 and time 2) from models and X-rays, i.e. 0.971, even the percentage increase in X-rays is about 13%. The purpose of this paper is to propose that, if repeatability is to be assessed, a funds control chart be established when a patient`s records are traced at the same time, which is a common practice.

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