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## 7.1 Hypothesis Tests and Confidence Intervals for a Single Coefficient

We will first discuss how to compute standard errors, test hypotheses and construct confidence intervals for a single regression coefficient \(\beta_j\) in a multiple regression model. The basic idea is summarized in Key Concept 7.1.

### Key Concept 7.1

###
Testing the Hypothesis \(\beta_j = \beta_{j,0}\)

Against the Alternative \(\beta_j \neq \beta_{j,0}\)

- Compute the standard error of \(\hat{\beta_j}\).
- Compute the \(t\)-statistic, \[t^{act} = \frac{\hat{\beta}_j - \beta_{j,0}} {SE(\hat{\beta_j})}\].
- Compute the \(p\)-value, \[p\text{-value} = 2 \Phi(-|t^{act}|)\]

`R`functions, e.g., by

`summary`.

Testing a single hypothesis about the significance of a coefficient in the multiple regression model proceeds similarly to the process in the simple regression model.

You can easily see this by inspecting the coefficient summary of the regression model

\[ TestScore = \beta_0 + \beta_1 \times STR \beta_2 \times english + u \]

already discussed in Chapter 6. Let us review this:

```
model <- lm(score ~ STR + english, data = CASchools)
coeftest(model, vcov. = vcovHC, type = "HC1")
#>
#> t test of coefficients:
#>
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) 686.032245 8.728225 78.5993 < 2e-16 ***
#> STR -1.101296 0.432847 -2.5443 0.01131 *
#> english -0.649777 0.031032 -20.9391 < 2e-16 ***
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
```

You can verify that these quantities are computed as in the simple regression model by manually calculating the \(t\)-statistics or \(p\)-values using the provided output above and using `R` as a calculator.

For example, using the definition of the \(p\)-value for a two-sided test as given in Key Concept 7.1, we can confirm the \(p\)-value for a test of the hypothesis that the coefficient \(\beta_1\), the coefficient on `size`, to be approximately zero.

```
# compute two-sided p-value
2 * (1 - pt(abs(coeftest(model, vcov. = vcovHC, type = "HC1")[2, 3]),
df = model$df.residual))
#> [1] 0.01130921
```

### Key Concept 7.2

### Confidence Intervals for a Single Coefficient in Multiple Regression

A \(95\%\) two-sided confidence interval for the coefficient \(\beta_j\) is an interval that contains the true value of \(\beta_j\) with a \(95 \%\) probability; that is, it contains the true value of \(\beta_j\) in \(95 \%\) of all repeated samples. Equivalently, it is the set of values of \(\beta_j\) that cannot be rejected by a \(5 \%\) two-sided hypothesis test. When the sample size is large, the \(95 \%\) confidence interval for \(\beta_j\) is \[\left[\hat{\beta_j}- 1.96 \times SE(\hat{\beta}_j), \hat{\beta_j} + 1.96 \times SE(\hat{\beta_j})\right].\]