Fitting a Quadratic in MINITAB^{TM}
Chapter 4, Printout 2
Section 1: Making a Fitted Quadratic Plot
 Launch MINITAB.
 Enter the data.
 Name Column C1 by clicking the blank column label and typing "x"
 Name Column C2 "y"
 In Column C1, enter the Ammonium Phosphate data from the "x" column in Table 4.3 (p. 133).
 In Column C2, enter the Compressive Strength data from the "y" column of Table 4.3.
Screenshot:
 Generate the fitted quadratic plot.
 Select "Stat > Regression > Fitted Line Plot."
 Fill out the resulting dialog box as follows:
Input Type 
Label 
Value 

Response (Y): 
y 

Predictor (X): 
x 

Quadratic 
selected 
Screenshot:
 Click "OK."
MINITAB displays a fitted quadratic plot of y vs. x.
Screenshot:
Section 2: Obtaining the Fitted Equation and Additional Numerical Summaries
 Add an x^{2} column.
 Name Column C3 by clicking the blank column label and typing "x^2"
 Select "Calc > Calculator."
 Fill out the resulting dialog box as follows:
Input Type 
Label 
Value 

Store result in variable: 
'x^2' 

Expression: 
'x'**2 
Screenshot:
 Click "OK."
Screenshot:
 Compute a quadratic fit and display summary information.
 Select "Stat > Regression > Regression."
 Fill out the resulting dialog box as follows:
Input Type 
Label 
Value 

Response: 
y 

Predictors: 
x 'x^2' 
Screenshot:
 Before closing the dialog box, click "Results."
 In the dialog box that follows, click the radio button next to the label, "In addition, the full table of fits and residuals."
Screenshot:
 Click "OK" to close the "Results" dialog box.
 Click "OK" in the "Regression" dialog box.
MINITAB displays summary information, including the regression equation and a table of fits and residuals.
Screenshot:
Section 3: Generating Additional Plots
 Display a normal plot of residuals, residuals vs. fits, and residuals vs. x.
 Select "Stat > Regression > Regression."
 Fill out the resulting dialog box as described in Section 2.
 Before closing the dialog box, click "Graphs."
 Fill out the resulting dialog box as follows:
Input Type 
Label 
Value 

Regular 
selected 

Histogram of residuals 
unchecked 

Normal plot of residuals 
checked 

Residuals versus fits 
checked 

Residuals versus order 
unchecked 

Residuals versus the variables: 

Screenshot:
 Click "OK" to close the "Graphs" dialog box.
 Click "OK" in the "Regression" dialog box.
MINITAB displays a normal plot of residuals, a plot of residuals vs. fits, and a plot of residuals vs. x.
Screenshot (normal plot of residuals):
Screenshot (residuals vs. fits):