Table 4.

Color change using WI and WI_D values, with data from the Vita Easyshade spectrophotometer measurements, for each group of treatment in different time points. The mean, standard deviation, and statistical analysis are displayed

Assessment points	Whiteness Index values			WId values
	37.5% Hydrogen Peroxide	6% Hydrogen Peroxide	Mann-Whitney test*	37.5% Hydrogen Peroxide	6% Hydrogen Peroxide	Mann-Whitney test*
Baseline	32.19 ± 2.88	31.79 ± 3.39	0.479	11.93 ± 6.04	12.23 ± 7.65	0.789
1-wk after bleaching	24.91 ± 2.85	27.63 ± 4.07	0.018	25.44 ± 4.74a	20.12 ± 7.83a	0.011
1-mth after bleaching	24.65 ± 2.94	28.20 ± 3.39	0.001	25.53 ± 6.62a	18.67 ± 6.87a	0.001
6-mths after bleaching	25.53 ± 2.63	29.31 ± 3.78	0.000	24.12 ± 4.80a	16.18 ± 6.86ab	0.000
12-mths after bleaching	24.89 ± 2.69**	27.81 ± 3.31**	0.003	24.26 ± 4.66a	18,96 ± 7.13a	0.003

* for comparison between both groups in each assessment time; ** for comparison between two assessment time (1-month vs 12-month after bleaching) in each group. No significant difference was found (Wilcoxon test; p > 0.05).** for comparison between two assessment time (1-month vs 12-month after bleaching) in each group. No significant difference was found (Student t test for paired sample; p > 0.05)