The following summary data on proportional stress limits for two different type of woods, Red oak and Douglas fir.
|
Type of Wood
|
Sample Size
|
Sample Mean
|
Sample
Standard Deviation
|
|
Red oak
|
50
|
8.51
|
1.52
|
|
Douglas fir
|
62
|
7.69
|
3.25
|
a) Find a 90% confidence interval for the difference between true average proportional stress limits for the Red oak and that for the Douglas fir. Interpret your result.
b) A test of hypotheses is conducted at ?=0.10 to determine if the stress limits are the same for the two type of woods. Explain how you can use the confidence interval in part (a) to draw a conclusion in the test of hypotheses.