## 3 Results  
In comparisons between groups at the start of the trial, parameters, L* and a* showed significant differences, but b* did not show significant differences (Table 1). All groups underwent significant color changes in L*, a*, and b* as a result of in-office bleaching between the initial and post-treatment evaluations. Table 2 shows differences in mean color parameters between initial and post-treatment evaluations. No significant differences between the four groups were found for the L* or a* parameters. For the b* parameter, the 35% HP group had a higher mean value than the 38% HP group. Comparing \(\Delta\)E between the four bleaching products (Table 2), no significant differences were found between them (Figure 1) (one-way ANOVA \(p=0.174\)). According to the univariate linear regression model with initial L* and a* values as covariables, while significant differences were found between groups at the initial valuation, adjusted \(\Delta\)E after treatment did not show significant differences between the four groups (\(p=0.577\)). The group presenting the highest \(\Delta\)E was 38% HP with a value of 4.63, while 40% HP gave a value of 4.01. When the whole sample was analyzed together, it was seen that \(\Delta\)E correlated negatively with initial L* value (Pearson = \(-0.292\)), whereby when the L* was higher, \(\Delta\)E was lower. However, L* was positively correlated with initial a* (Pearson = 0.267) and b* values (Pearson = 0.212) (Figure 2). \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **35\% H2O2 + Light** & **37.5\% H2O2 Mean** & **38\% H2O2 Mean** & **40\% H2O2 Mean** & **Inter-Group** \\  & **Mean (CI-95\%)** & **(CI-95\%)** & **(CI-95\%)** & **(CI-95\%)** & **ANOVA Test** \\ \hline _Difference in L*_ & 2.92 (2.11, 3.72) & 2.32 (1.48, 3.15) & 1.84 (1.30, 2.37) & 2.17 (1.20, 3.16) & \(p=0.286\) \\ \hline _Difference in a*_ & \(-0.26\) (\(-0.46\), \(-0.06\)) & \(-0.34\) (\(-0.51\), \(-0.17\)) & \(-0.40\) (\(-0.63\), \(-0.17\)) & \(-0.11\) (\(-0.28\), \(-0.06\)) & \(p=0.172\) \\ \hline _Difference in b*_ & \(-1.11\) (\(-1.90\), \(-0.31\)) & \(-2.39\) (\(-2.95\), \(-1.84\)) & \(-2.57\) (\(-3.46\), \(-1.68\)) & \(-1.23\) (\(-1.81\), \(-0.64\)) & \(p=0.004\) post hoc \\  & & & & & \\ \hline \(\Delta\)E & 4.73 (4.05, 5.42) & 4.57 (3.97, 5.17) & 4.26 (3.52, 5.01) & 3.76 (3.13, 4.40) & \(p=0.174\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Differences in mean L*, a*, and b* parameters and \(\Delta\)E from initial to final evaluations. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **35\% H2O2 + Light** & **37.5\% H2O2 Mean** & **38\% H2O2 Mean** & **40\% H2O2 Mean** & **Inter-Group** \\  & **Mean (CI-95\%)** & **(CI-95\%)** & **(CI-95\%)** & **(CI-95\%)** & **ANOVA Test** \\ \hline _Difference in L*_ & 2.92 (2.11, 3.72) & 2.32 (1.48, 3.15) & 1.84 (1.30, 2.37) & 2.17 (1.20, 3.16) & \(p=0.286\) \\ \hline _Difference in a*_ & \(-0.26\) (\(-0.46\), \(-0.06\)) & \(-0.34\) (\(-0.51\), \(-0.17\)) & \(-0.40\) (\(-0.63\), \(-0.17\)) & \(-0.11\) (\(-0.28\), \(-0.06\)) & \(p=0.172\) \\ \hline _Difference in b*_ & \(-1.11\) (\(-1.90\), \(-0.31\)) & \(-2.39\) (\(-2.95\), \(-1.84\)) & \(-2.57\) (\(-3.46\), \(-1.68\)) & \(-1.23\) (\(-1.81\), \(-0.64\)) & \(p=0.004\) post hoc \\  & & & & & \\ \hline \(\Delta\)E & 4.73 (4.05, 5.42) & 4.57 (3.97, 5.17) & 4.26 (3.52, 5.01) & 3.76 (3.13, 4.40) & \(p=0.174\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Initial and final L*, a*, and b* values in the four study groups. Figure 1: Non-adjusted comparison of initial \(\Delta\)E and initial adjusted L* and a* between groups. Figure 2: _Cont._  
In the forward stepwise linear regression model (R squared 0.149; \(p<0.001\)), initial \(\Delta\)E was the dependent variable; initial parameters L*, a*, and b* were independent variables; and treatment groups were dummy variables. For this model, only initial L* and b* values were found to be significantly predictive variables (\(p<0.001\)).