Question

Twenty-one daily responses of stack loss (y) (the amount of ammonia escaping) were measured with air flow x1, temperature x2, and acid concentration x3.

1. Check the significance of the model using ANOVA via Minitab and state your conclusion at 5% alpha level. [5 marks]

2. Using stepwise method, find the best regression model by Minitab and explain the results obtained at each step. [3 marks]

y	x1	x2	x3
42	80	27	89
37	80	27	88
37	75	25	90
28	62	24	87
18	62	22	87
18	62	23	87
19	62	24	93
20	62	24	93
15	58	23	87
14	58	18	80
14	58	18	89
13	58	17	88
11	58	18	82
12	58	19	93
8	50	18	89
7	50	18	86
8	50	19	72
8	50	19	79
9	50	20	80
15	56	20	82
15	70	20	91

Answer 1

Using Minitab

From ANOVA F value = 89.64 and p value =0.000 therefore model is significant.
Also p-value corresponding to x3 is 0.344 > 0.05 therefore x3 is not significant.
We delete x3 from the model and refit the model then observe that p value corresponding to x1, x2 are 0.000 and 0.002 x1 and x2 are statistically significant.