Fig. 2c is the response surface and contour plots showing the effects of extraction time (X2) and ratio of solvent to material (X3) on the yield of flavonoids. The effect of extraction time on the flavonoids yield is shown in Fig. 2a. Again, these results indicated that both factors have slight effects on the total flavonoids yield, which is the same as the results in Table 2. The interaction effect was not significant (p value of x3x2 = 0.71). Fig. 2d described the interaction effects of the ratio of solvent to material (X3) and ethanol concentration (X4). The effect of ratio of solvent to material on the yield of total flavonoids displayed a linear increase when concentration of ethanol was lower while decrease when concentration of ethanol was higher. So a lower level of ethanol and higher ratio of solvent to material can result in a higher yield of total flavonoids