Concept explainers
a.
Identify and explain whichof the given modelscan be recommended.
a.

Answer to Problem 66SE
The model with 2 predictors and the model with 3 predictors can be recommended for predicting the pH before addition of dyes.
Explanation of Solution
Given info:
The MINITAB output shows the best regression option for the data predicted for pH before the addition of dyes using carpet density, carpet weight, dye weight, dye weight as a percentage of carpet and pH after addition of dyes.
Justification:
Mallows
It is used to assess the fit of regression model where the aim to find the best subset of predictors. A relatively small value of
By observing the mallows
By examining the models with three variables,
Hence, the model with two predictorsnamely dye weight and pH after addition of dyes could be considered as a best model subset for predicting pH before the addition of dyes.
Also, a second option would be the model with three predictorsnamely carpet weight, dye weight and pH after addition of dyes could be considered as a best model subset for predicting pH before the addition of dyes.
b.
Test whether the model suggests a useful linear relationship between pH before the addition of dyes and at least one of the predictors.
b.

Answer to Problem 66SE
There is sufficient evidence to conclude that the there is a use of linear relationship between pH before the addition of dyes and at least one of the predictors dye weight and pH after the addition of dyes.
Explanation of Solution
Given info:
The MINITAB output for predicting the pH before the addition of dyes using the dye weight
Calculation:
The test hypotheses are given below:
Null hypothesis:
That is, there is no use of linear relationship between pH before the addition of dyes and the predictors dye weightand pH after the addition of dyes.
Alternative hypothesis:
That is, there is a use of linear relationship between pH before the addition of dyes and at least one of the predictors dye weightand pH after the addition of dyes.
Conclusion:
The P-value is 0.000 and the level of significance is 0.001.
The P-value is lesser than the level of significance.
That is
Thus, the null hypothesis is rejected.
Hence, there is sufficient evidence to conclude that there is a use of linear relationship between pH before the addition of dyes and at least one of the predictors dye weight and pH after the addition of dyes.
c.
Explain whether either one of the predictors could be eliminated from the model given that the other predictor is retained.
c.

Answer to Problem 66SE
No, either one of the predictors could not be eliminated from the model given that the other predictor is retained.
Explanation of Solution
Calculation:
For variable
Testing the hypothesis:
Null hypothesis:
That is, there is no use of linear relationship between pH before the addition of dyes and dye weightgiven that pH after addition of dyes was retained in the model.
Alternative hypothesis:
That is, there is a use of linear relationship between pH before the addition of dyes and dye weightgiven that pH after addition of dyes was retained in the model.
From the MINITAB output it can be observed that the P-value corresponding to the t statistic of
Conclusion:
The P-value is 0.000 and the level of significance is 0.001.
The P-value is lesser than the level of significance.
That is
Thus, the null hypothesis is rejected.
Hence, there is sufficient evidence to conclude that there is a use of linear relationship between pH before the addition of dyes and dye weight given that pH after addition of dyes was retained in the model.
For variable
Testing the hypothesis:
Null hypothesis:
That is, there is no use of linear relationship between pH before the addition of dyes and pH after addition of dyes given that dye weight was retained in the model.
Alternative hypothesis:
That is, there is a use of linear relationship between pH before the addition of dyes and pH after addition of dyes given that dye weight was retained in the model.
From the MINITAB output it can be observed that the P-value corresponding to the t statistic of
Conclusion:
The P-value is 0.000 and the level of significance is 0.001.
The P-value is lesser than the level of significance.
That is
Thus, the null hypothesis is rejected.
Hence, there is sufficient evidence to conclude that there is a use of linear relationship between pH before the addition of dyes and pH after addition of dyes given that dye weight was retained in the model.
Justification:
From the analysis it can be concluded that none of the variables can be eliminated from the model given that the other variable is already present in the model.
d.
Calculate and interpret the 95% confidence interval for the two predictors.
d.

Answer to Problem 66SE
The 95% confidence interval for the estimated slope coefficient
(–0.0000684, –0.0000244).
The 95% confidence interval for the estimated slope coefficient
Explanation of Solution
Calculation:
The 95% confidence interval is calculated using the formula:
The confidence interval is calculated using the formula:
Where,
n is the total number of observations.
k is the total number of predictors in the model.
Critical value:
Software procedure:
Step-by-step procedure to find the critical value is given below:
- Click on Graph, select View Probability and click OK.
- Select t, enter 111 as Degrees of freedom, inShaded Area Tab select Probability under Define Shaded Area By and choose Both tails.
- Enter Probability value as 0.05.
- Click OK.
Output obtained from MINITAB is given below:
The 95% confidence interval for
Thus, the 95% confidence interval for the estimated slope coefficient
(–0.0000684, –0.0000244).
The 95% confidence interval for
Thus, the 95% confidence interval for the estimated slope coefficient
(0.6417,0.8325).
Interpretation:
For the variable
For one unit increase in the dye weight, it is 95% confident that the estimated value of pH before addition of dyes would decrease between–0.00000684 and–0.0000244 given that pH after addition of dyes is fixed constant.
For the variable
For one unit increase in the pH after the addition of dyes it is 95% confident that the estimated value of pH before addition of dyes would increase between 0.6417 and 0.8325 given that dye weight is fixed constant.
e.
Calculate and interpret the 95% confidence interval for the average value of pH before the addition of dyes when the dye weight and pH after the addition of dyes takes 1,000 and 6, respectively.
e.

Answer to Problem 66SE
The 95% confidence interval for the average value of pH before the addition of dyes when the dye weight and pH after the addition of dyes takes 1,000 and 6, respectively is (5.250, 5.383)
Explanation of Solution
Given info:
The estimated standard deviation for predicting the pH before the addition of dyes when the dye weight and pH after the addition of dyes takes 1,000 and 6 is 0.0336.
Calculation:
The average value of pH before the addition of dyes when the dye weight and pH after the addition of dyes takes 1,000 and 6 is calculated as follows:
Thus, the average value of pH before the addition of dyes when the dye weight and pH after the addition of dyes takes 1,000 and 6 is 5.316.
95% confidence interval for the true response:
The confidence interval is calculated using the formula:
Where,
n is the total number of observations.
k is the total number of predictors in the model.
Critical value:
Software procedure:
Step-by-step procedure to find the critical value is given below:
- Click on Graph, select View Probability and click OK.
- Select t, enter 111 as Degrees of freedom, in Shaded Area Tab select Probability under Define Shaded Area By and choose Both tails.
- Enter Probability value as 0.05.
- Click OK.
Output obtained from MINITAB is given below:
The 95% confidence interval is given below:
Thus, the 95% confidence interval for the average value of pH before the addition of dyes when the dye weight and pH after the addition of dyes takes 1,000 and 6 is (5.250,5.383).
Interpretation:
It is 95% confident that average value of pH before the addition of dyes when the dye weight and pH after the addition of dyes takes 1,000 and 6 would lie between 5.250 and 5.383.
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Chapter 13 Solutions
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