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Remember that this estimate of σ² is model dependent. Note that this differs slightly from the value given in the Minitab output (Table 2.3) because of rounding.

2.2.4 Alternate Form of the Model

      There is an alternate form of the simple linear regression model that is occasionally useful. Suppose that we redefine the regressor variable xi as the deviation from its own average, say . The regression model then becomes

(2.20)

Note that redefining the regressor variable in Eq. (2.20) has shifted the origin of the x’s from zero to . In order to keep the fitted values the same in both the original and transformed models, it is necessary to modify the original intercept. The relationship between the original and transformed intercept is

      (2.21)

      It is easy to show that the least-squares estimator of the transformed intercept is . The estimator of the slope is unaffected by the transformation. This alternate form of the model has some advantages. First, the least-squares estimators and are uncorrelated, that is, . This will make some applications of the model easier, such as finding confidence intervals on the mean of y (see Section 2.4.2). Finally, the fitted model is

(2.22)

Although Eqs. (2.22) and (2.8) are equivalent (they both produce the same value of for the same value of x), Eq. (2.22) directly reminds the analyst that the regression model is only valid over the range of x in the original data. This region is centered at .

2.3 HYPOTHESIS TESTING ON THE SLOPE AND INTERCEPT

      We are often interested in testing hypotheses and constructing confidence intervals about the model parameters. Hypothesis testing is discussed in this section, and Section 2.4 deals with confidence intervals. These procedures require that we make the additional assumption that the model errors εi are normally distributed. Thus, the complete assumptions are that the errors are normally and independently distributed with mean 0 and variance σ², abbreviated NID(0, σ²). In Chapter 4 we discuss how these assumptions can be checked through residual analysis.

      2.3.1 Use of t Tests

      Suppose that we wish to test the hypothesis that the slope equals a constant, say β₁₀. The appropriate hypotheses are

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