A multiple regression analysis was carried out to relate y

tensile strength of a synthetic-fiber specimen to the variables x1
percent cotton and x2
drying time. The data set consisted of n
12 observations.

a. The estimated coefficients were ˆ

0
180.00, ˆ

1
1.000, and ˆ

2
10.500. Calculate a point estimate of the expected tensile strength when percent cotton
15 and drying time
3.5.

b. Sums of squares were SST
1210.30 and SSE

117.40. What proportion of observed variation in tensile strength can be attributed to the model relationship?

c. Use the information in part

(b) to decide whether the model with variables x1 and x2 specifies a useful relationship.

d. The estimated standard deviation of ˆ 0 ˆ 1x1 ˆ 2x2 when x1
18 and x2
3.0 was 1.20. Calculate a 95%

PI for tensile strength of a fabric specimen for which x1
18 and x2
3.0.