Table 3.

Central composite response surface methodology (RSM) and experimental data.

Trials	X1: Ethanol Concentration/%	X2: Solid/Solvent Ratio (g/mL)	X3: Temperature/°C	X4:Extraction Time/min	Y: Betulin Yield/%
1	0	1	0	−1	89.25
2	0	−1	−1	0	87.25
3	1	1	0	0	91.25
4	0	−1	0	1	89.50
5	0	0	0	0	92.75
6	1	0	−1	0	90.00
7	0	0	0	0	92.75
8	0	1	−1	0	89.75
9	0	−1	0	−1	88.75
10	0	1	1	0	90.50
11	−1	0	0	1	89.75
12	−1	0	−1	0	89.25
13	0	0	0	0	92.75
14	1	0	0	−1	90.75
15	−1	1	0	0	87.25
16	0	0	0	0	92.75
17	-1	0	1	0	90.00
18	0	0	1	−1	91.75
19	0	0	−1	−1	88.25
20	0	0	0	0	92.75
21	−1	−1	0	0	89.50
22	1	0	0	1	89.25
23	1	−1	0	0	86.50
24	0	0	−1	1	90.75
25	0	−1	1	0	89.75
26	0	1	0	1	91.00
27	0	0	1	1	89.50
28	−1	0	0	−1	89.75
29	1	0	1	0	89.75