Table 1. Factors, Levels, and Extractions at Various Levels of Factors.

factors	levels
	+1	0	-1
HCl concentration (X1), % v/v	10	6	2
time (X2), h	4	2.5	1
temperature (X3), °C	80	65	50

experimental results
levels			extraction (%)
X1	X2	X3	Sm	Co
10	1	80	86.72	85.81
10	4	80	88.59	99.96
10	1	50	40.50	53.13
10	4	50	61.17	60.11
2	1	80	18.19	17.54
2	4	80	42.36	42.48
2	1	50	15.61	16.39
2	4	50	31.48	25.65
Base Level Experiments
6	2.5	65	50.75	46.91
6	2.5	65	50.23	47.46
6	2.5	65	50.32	51.09