Linearity of JND and stimulus text lengths. Comparisons of the standard deviations σ for for the represented lengths for each tester, serving as measure of JND, as function of the test text lengths . Lines show best-fit values using linear functions corresponding to Weber law. In all individual cases the linear regression provides a good fit, , but the accuracy of the detection, measured by β, varied strongly between the testers. For each tester we present the value of the linear regression coefficient β and the coefficient of determination . The linear relationship between the standard deviation and (Weber law) is well preserved with the coefficients of determination higher than 0.9 for eight of the nine subjects and above 0.8 for the remaining one.