Figure. Correlation between the surface roughness and the mean failure stress of the 4 investigated ceramic veneering materials of the ceramic systems Empress 1 (a), Empress 2 (b), Symbio Ceram (c), and Vita Akzent (d). The data points indicate the mean values of each surface quality, respectively (n = 20 for each data point). The bars indicate the standard deviations of the failure stress and the surface roughness values, respectively.