Table 4.  Means and standard deviation values for surface roughness for all subgroups of the two composites in shade A2.
Variable   Material   Bleach   Mean   SD   Regression analysis for materials   Regression analysis for bleach agents
After first bleach  Durafil VS  Control  126.1  5.3  0.011*  0.39
Beyond  137.6  13.2
Luma  131  13.1
Opal  127.5  6.5
TPH3  Control  150.1  9.7
Beyond  125.6  6.5
Luma  143.3  6.6
Opal  138  5.7
After second bleach  Durafil VS  Control  126.1  5.3  <0.001*  <0.001*
*:  Significant at P≤.05