# Fitting a Quadratic in MINITABTM

## Chapter 4, Printout 2

### Section 1: Making a Fitted Quadratic Plot

1. Launch MINITAB.

2. 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.

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3. 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

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• Click "OK."

MINITAB displays a fitted quadratic plot of y vs. x.

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### Section 2: Obtaining the Fitted Equation and Additional Numerical Summaries

• 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

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• Click "OK."

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2. 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'

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• 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."

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• 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.

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### Section 3: Generating Additional Plots

1. 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:

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• 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):