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Applied Statistics Using SPSS, STATISTICA, MATLAB and R

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296 7 Data Regression<br />

7.2.5 ANOVA <strong>and</strong> Extra Sums of Squares<br />

The simple ANOVA test presented in 7.1.4, corresponding to the decomposition of<br />

the total sum of squares as expressed by formula 7.24, can be generalised in a<br />

straightforward way to the multiple regression model.<br />

Example 7.14<br />

Q: Apply the simple ANOVA test to the foetal weight regression in Example 7.13.<br />

A: Table 7.2 lists the results of the simple ANOVA test, obtainable with <strong>SPSS</strong><br />

<strong>STATISTICA</strong>, or R, for the foetal weight data, showing that the regression model<br />

is statistically significant ( p ≈ 0).<br />

Table 7.2. ANOVA test for Example 7.13.<br />

Sum of<br />

Squares<br />

df<br />

Mean<br />

Squares<br />

F p<br />

SSR 128252147 3 42750716 501.9254 0.00<br />

SSE 34921110 410 85173<br />

SST 163173257<br />

It is also possible to apply the ANOVA test for lack of fit in the same way as<br />

was done in 7.1.4. However, when there are several predictor values playing their<br />

influence in the regression model, it is useful to assess their contribution by means<br />

of the so-called extra sums of squares. An extra sum of squares measures the<br />

marginal reduction in the error sum of squares when one or several predictor<br />

variables are added to the model.<br />

We now illustrate this concept using the foetal weight data. Table 7.3 shows the<br />

regression lines, SSE <strong>and</strong> SSR for models with one, two or three predictors. Notice<br />

how the model with (BPD,CP) has a decreased error sum of squares, SSE, when<br />

compared with either the model with BPD or CP alone, <strong>and</strong> has an increased<br />

regression sum of squares. The same happens to the other models. As one adds<br />

more predictors one expects the linear fit to improve. As a consequence, SSE <strong>and</strong><br />

SSR are monotonic decreasing <strong>and</strong> increasing functions, respectively, with the<br />

number of variables in the model. Moreover, what SSE decreases is reflected by an<br />

equal increase of SSR.<br />

We now define the following extra sum of squares, SSR(X2|X1), which measures<br />

the improvement obtained by adding a second variable X2 to a model that has<br />

already X1:<br />

SSR(X2 | X1) = SSE(X1) − SSE(X1, X2) = SSR(X1, X2) − SSR(X1).

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