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Multiple Linear Regression

The general purpose of multiple regression (the term was first used by Pearson, 1908), as a generalization of simple linear regression, is to learn about how several independent variables or predictors (IVs) together predict a dependent variable (DV). Multiple regression analysis often focuses on understanding (1) how much variance in a DV a set of IVs explain and (2) the relative predictive importance of IVs in predicting a DV.

In the social and natural sciences, multiple regression analysis is very widely used in research. Multiple regression allows a researcher to ask (and hopefully answer) the general question "what is the best predictor of ...". For example, educational researchers might want to learn what the best predictors of success in college are. Psychologists may want to determine which personality dimensions best predicts social adjustment.

Multiple regression model

A general multiple linear regression model at the population level can be written as

\[y_{i}=\beta_{0}+\beta_{1}x_{1i}+\beta_{2}x_{2i}+\ldots+\beta_{k}x_{ki}+\varepsilon_{i} \]

Parameter estimation

The least squares method used for the simple linear regression analysis can also be used to estimate the parameters in a multiple regression model. The basic idea is to minimize the sum of squared residuals or errors. Let $b_{0},b_{1},\ldots,b_{k}$ represent the estimated regression coefficients.The individual $i$'s residual $e_{i}$ is the difference between the observed $y_{i}$ and the predicted $y_{i}$

\[ e_{i}=y_{i}-\hat{y}_{i}=y_{i}-b_{0}-b_{1}x_{1i}-\ldots-b_{k}x_{ki}.\]

The sum of squared residuals is

\[ SSE=\sum_{i=1}^{n}e_{i}^{2}=\sum_{i=1}^{n}(y_{i}-\hat{y}_{i})^{2}. \]

By minimizing $SSE$, the regression coefficient estimates can be obtained as

\[ \boldsymbol{b}=(\boldsymbol{X}'\boldsymbol{X})^{-1}\boldsymbol{X}'\boldsymbol{y}=(\sum\boldsymbol{x}_{i}\boldsymbol{x}_{i}')^{-1}(\sum\boldsymbol{x}_{i}\boldsymbol{y}_{i}). \]

\(R^2\)

How well the multiple regression model fits the data can be assessed using the $R^{2}$. Its calculation is the same as for the simple regression

\[\begin{align*} R^{2} & = & 1-\frac{\sum e_{i}^{2}}{\sum_{i=1}^{n}(y_{i}-\bar{y})^{2}}\\& = & \frac{\text{Variation explained by IVs}}{\text{Total variation}} \end{align*}. \]

In multiple regression, $R^{2}$ is the total proportion of variation in $y$ explained by the multiple predictors.

The $R^{2}$ increases or at least is the same with the inclusion of more predictors. However, with more predators, the model becomes more complex and potentially more difficult to interpret. In order to take into consideration of the model complexity, the adjusted $R^{2}$ has been defined, which is calculated as

\[aR^{2}=1-(1-R^{2})\frac{n-1}{n-k-1}.\]

Hypothesis testing of regression coefficient(s)

With the estimates of regression coefficients and their standard errors estimates, we can conduct hypothesis testing for one, a subset, or all regression coefficients.

Testing a single regression coefficient

At first, we can test the significance of the coefficient for a single predictor. In this situation, the null and alternative hypotheses are

\[ H_{0}:\beta_{j}=0\text{ vs }H_{1}:\beta_{j}\neq0 \]

with $\beta_{j}$ denoting the regression coefficient of $x_{j}$ at the population level.

As in the simple regression, we use a test statistic

\[ t_{j}=\frac{b_{j} - \beta{j} }{s.e.(b_{j})}\]

where $b_{j}$ is the estimated regression coefficient of $x_{j}$ using data from a sample. If the null hypothesis is true and $\beta_j = 0$, the test statistic follows a t-distribution with degrees of freedom \(n-k-1\) where \(k\) is the number of predictors.

One can also test the significance of \(\beta_j\) by constructing a confidence interval for it. Based on a t distribution, the \(100(1-\alpha)%\) confidence interval is

\[ [b_{j}+t_{n-k-1}(\alpha/2)*s.e.(b_{j}),\;b_{j}+t_{n-k-1}(1-\alpha/2)*s.e.(b_{j})]\]

where $t_{n-k-1}(\alpha/2)$ is the $\alpha/2$ percentile of the t distribution. As previously discussed, if the confidence interval includes 0, the regression coefficient is not statistically significant at the significance level $\alpha$.

Testing all the regression coefficients together (overall model fit)

Given the multiple predictors, we can also test whether all of the regression coefficients are 0 at the same time. This is equivalent to test whether all predictors combined can explained a significant portion of the variance of the outcome variable. Since $R^2$ is a measure of the variance explained, this test is naturally related to it.

For this hypothesis testing, the null and alternative hypothesis are

\[H_{0}:\beta_{1}=\beta_{2}=\ldots=\beta_{k}=0\]

vs.

\[H_{1}:\text{ at least one of the regression coefficients is different from 0}.\]

In this kind of test, an F test is used. The F-statistic is defined as

\[F=\frac{n-k-1}{k}\frac{R^{2}}{1-R^{2}}.\]

It follows an F-distribution with degrees of freedom $k$ and $n-k-1$ when the null hypothesis is true. Given an F statistic, its corresponding p-value can be calculated from the F distribution as shown below. Note that we only look at one side of the distribution because the extreme values should be on the large value side.

R2

Testing a subset of the regression coefficients

We can also test whether a subset of $p$ regression coefficients, e.g., $p$ from 1 to the total number coefficients $k$, are equal to zero. For convenience, we can rearrange all the $p$ regression coefficients to be the first $p$ coefficients. Therefore, the null hypothesis should be

\[H_{0}:\beta_{1}=\beta_{2}=\ldots=\beta_{p}=0\]

and the alternative hypothesis is that at least one of them is not equal to 0.

As for testing the overall model fit, an F test can be used here. In this situation, the F statistic can be calculated as

\[F=\frac{n-k-1}{p}\frac{R^{2}-R_{0}^{2}}{1-R^{2}},\]

which follows an F-distribution with degrees of freedom $p$ and $n-k-1$. $R^2$ is for the regression model with all the predictors and $R_0^2$ is from the regression model without the first $p$ predictors $x_{1},x_{2},\ldots,x_{p}$ but with the rest predictors $x_{p+1},x_{p+2},\ldots,x_{k}$.

Intuitively, this test determine whether the variance explained by the first \(p\) predictors above and beyond the $k-p$ predictors is significance or not. That is also the increase in R-squared.

An example

As an example, suppose that we wanted to predict student success in college. Why might we want to do this? There's an ongoing debate in college and university admission offices (and in the courts) regarding what factors should be considered important in deciding which applicants to admit. Should admissions officers pay most attention to more easily quantifiable measures such as high school GPA and SAT scores? Or should they give more weight to more subjective measures such as the quality of letters of recommendation? What are the pros and cons of the approaches? Of course, how we define college success is also an open question. For the sake of this example, let's measure college success using college GPA.

In this example, we use a set of simulated data (generated by us). The data are saved in the file gpa.csv. As shown below, the sample size is 100 and there are 4 variables: college GPA (c.gpa), high school GPA (h.gpa), SAT, and quality of recommendation letters (recommd).

> usedata('gpa')
> dim(gpa)
[1] 100   4
> head(gpa)
  c.gpa h.gpa  SAT recommd
1  2.04  2.01 1070       5
2  2.56  3.40 1254       6
3  3.75  3.68 1466       6
4  1.10  1.54  706       4
5  3.00  3.32 1160       5
6  0.05  0.33  756       3
> 

Graph the data

Before fitting a regression model, we should check the relationship between college GPA and each predictor through a scatterplot. A scatterplot can tell us the form of relationship, e.g., linear, nonlinear, or no relationship, the direction of relationship, e.g., positive or negative, and the strength of relationship, e.g., strong, moderate, or weak. It can also identify potential outliers.

The scatterplots between college GPA and the three potential predictors are given below. From the plots, we can roughly see all three predictors are positively related to the college GPA. The relationship is close to linear and the relationship seems to be stronger for high school GPA and SAT than for the quality of recommendation letters.

> usedata('gpa')
> attach(gpa)
> 
> par(mfrow=c(2,2))
> plot(h.gpa, c.gpa)
> plot(SAT, c.gpa)
> plot(recommd, c.gpa)
> 
R plot

Descriptive statistics

Next, we can calculate some summary statistics to explore our data further. For each variable, we calculate 6 numbers: minimum, 1st quartile, median, mean, 3rd quartile, and maximum. Those numbers can be obtained using the summary() function. To look at the relationship among the variables, we can calculate the correlation matrix using the correlation function cor().

Based on the correlation matrix, the correlation between college GPA and high school GPA is about 0.545, which is larger than that (0.523) between college GPA and SAT, in turn larger than that (0.35) between college GPA and quality of recommendation letters.

> usedata('gpa')
> summary(gpa)
     c.gpa           h.gpa            SAT          recommd     
 Min.   :0.050   Min.   :0.330   Min.   : 400   Min.   : 2.00  
 1st Qu.:1.562   1st Qu.:1.640   1st Qu.: 852   1st Qu.: 4.00  
 Median :1.985   Median :1.930   Median :1036   Median : 5.00  
 Mean   :1.980   Mean   :2.049   Mean   :1015   Mean   : 5.19  
 3rd Qu.:2.410   3rd Qu.:2.535   3rd Qu.:1168   3rd Qu.: 6.00  
 Max.   :4.010   Max.   :4.250   Max.   :1500   Max.   :10.00  
> cor(gpa)
            c.gpa     h.gpa       SAT   recommd
c.gpa   1.0000000 0.5451980 0.5227546 0.3500768
h.gpa   0.5451980 1.0000000 0.4326248 0.6265836
SAT     0.5227546 0.4326248 1.0000000 0.2175928
recommd 0.3500768 0.6265836 0.2175928 1.0000000
> 

Fit a multiple regression model

As for the simple linear regression, The multiple regression analysis can be carried out using the lm() function in R. From the output, we can write out the regression model as

\[ c.gpa = -0.153+ 0.376 \times h.gpa + 0.00122 \times SAT + 0.023 \times recommd \]

> usedata('gpa')
> gpa.model<-lm(c.gpa~h.gpa+SAT+recommd, data=gpa)
> summary(gpa.model)

Call:
lm(formula = c.gpa ~ h.gpa + SAT + recommd, data = gpa)

Residuals:
    Min      1Q  Median      3Q     Max 
-1.0979 -0.4407 -0.0094  0.3859  1.7606 

Coefficients:
              Estimate Std. Error t value Pr(>|t|)    
(Intercept) -0.1532639  0.3229381  -0.475 0.636156    
h.gpa        0.3763511  0.1142615   3.294 0.001385 ** 
SAT          0.0012269  0.0003032   4.046 0.000105 ***
recommd      0.0226843  0.0509817   0.445 0.657358    
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Residual standard error: 0.5895 on 96 degrees of freedom
Multiple R-squared:  0.3997,	Adjusted R-squared:  0.381 
F-statistic: 21.31 on 3 and 96 DF,  p-value: 1.16e-10

> 

Interpret the results / output

From the output, we see the intercept is -0.153. Its immediate meaning is that when all predictors' values are 0, the predicted college GPA is -0.15. This clearly does not make much sense because one would never get a negative GPA, which results from the unrealistic presumption that the predictors can take the value of 0.

The regression coefficient for the predictor high school GPA (h.gpa) is 0.376. This can be interpreted as keeping SAT and recommd scores constant, the predicted college GPA would increase 0.376 with a unit increase in high school GPA.This is again might be problematic because it might be impossible to increase high school GPA while keeping the other two predictors unchanged. The other two regression coefficients can be interpreted in the same way.

From the output, we can also see that the multiple R-squared ($R^2$) is 0.3997. Therefore, about 40% of the variation in college GPA can be explained by the multiple linear regression with h.GPA, SAT, and recommd as the predictors. The adjusted $R^2$ is slightly smaller because of the consideration of the number of predictors. In fact,

\[ \begin{eqnarray*} aR^{2} & = & 1-(1-R^{2})\frac{n-1}{n-k-1}\\& = & 1-(1-.3997)\frac{100-1}{100-3-1}\\& = & .3809 \end{eqnarray*} \]

Hypothesis testing

Testing Individual Regression Coefficient

For any regression coefficients for the three predictors (also the intercept), a t test can be conducted. For example, for high school GPA, the estimated coefficient is 0.376 with the standard error 0.114. Therefore, the corresponding t statistic is \(t = 0.376/0.114 = 3.294\). Since the statistic follows a t distribution with the degrees of freedom \(df = n - k - 1 = 100 - 3 -1 =96\), we can obtain the p-value as \(p = 2*(1-pt(3.294, 96))= 0.0013\). Since the p-value is less than 0.05, we conclude the coefficient is statistically significant. Note the t value and p-value are directly provided in the output.

> t <- 0.376/0.114
> t
[1] 3.298246
> 2*(1-pt(t, 96))
[1] 0.001365401
> 

Overall model fit (testing all coefficients together)

To test all coefficients together or the overall model fit, we use the F test. Given the $R^2$, the F statistic is

\[ \begin{eqnarray*} F & = & \frac{n-k-1}{k}\frac{R^{2}}{1-R^{2}}\\ & = & \left(\frac{100-3-1}{3}\right)\times \left(\frac{0.3997}{1-.3997}\right )=21.307\end{eqnarray*} \]

which follows the F distribution with degrees of freedom $df1=k=3$ and $df2=n-k-1=96$. The corresponding p-value is 1.160e-10. Note that this information is directly shown in the output as "F-statistic: 21.31 on 3 and 96 DF, p-value: 1.160e-10".

Therefore, at least one of the regression coefficients is statistically significantly different from 0. Overall, the three predictors explained a significant portion of the variance in college GPA. The regression model with the 3 predictors is significantly better than the regression model with intercept only (i.e., predict c.gpa by the mean of c.gpa).

> F <- (100 - 3 -1)/3*0.3997/(1-0.3997)
> F
[1] 21.30668
> 1 - pf(F, 3, 96)
[1] 1.162797e-10
> 

Testing a subset of regression coefficients

Suppose we are interested in testing whether the regression coefficients of high school GPA and SAT together are significant or not. Alternative, we want to see above and beyond the quality of recommendation letters, whether the two predictors can explain a significant portion of variance in college GPA. To conduct the test, we need to fit two models:

From the full model, we can get the $R^2 = 0.3997$ with all three predictors and from the reduced model, we can get the $R_0^2 = 0.1226$ with only quality of recommendation letters. Then the F statistic is constructed as

\[F=\frac{n-k-1}{p}\frac{R^{2}-R_{0}^{2}}{1-R^{2}}=\left(\frac{100-3-1}{2}\right )\times\frac{.3997-.1226}{1-.3997}=22.157.\]

Using the F distribution with the degrees of freedom $p=2$ (the number of coefficients to be tested) and $n-k-1 = 96$, we can get the p-value close to 0 ($p=1.22e-08$).

> usedata('gpa')
> model.reduce <- lm(c.gpa~recommd, data=gpa)
> summary(model.reduce)

Call:
lm(formula = c.gpa ~ recommd, data = gpa)

Residuals:
     Min       1Q   Median       3Q      Max 
-1.90257 -0.33372  0.01973  0.43457  1.71204 

Coefficients:
            Estimate Std. Error t value Pr(>|t|)    
(Intercept)  1.07020    0.25596   4.181 6.31e-05 ***
recommd      0.17539    0.04741   3.700 0.000356 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Residual standard error: 0.7054 on 98 degrees of freedom
Multiple R-squared:  0.1226,	Adjusted R-squared:  0.1136 
F-statistic: 13.69 on 1 and 98 DF,  p-value: 0.0003564

> 
> F <- (100 - 3 -1)/2*(0.3997-0.1226)/(1-0.3997)
> F
[1] 22.15692
> 1 - pf(F, 2, 96)
[1] 1.225164e-08
> 

Note that the test conducted here is based on the comparison of two models. In R, if there are two models, they can be compared conveniently using the R function anova(). As shown below, we obtain the same F statistic and p-value.

> usedata('gpa')
> model.full <- lm(c.gpa~h.gpa+SAT+recommd, data=gpa)
> model.reduce <- lm(c.gpa~recommd, data=gpa)
> anova(model.reduce, model.full)
Analysis of Variance Table

Model 1: c.gpa ~ recommd
Model 2: c.gpa ~ h.gpa + SAT + recommd
  Res.Df    RSS Df Sum of Sq      F    Pr(>F)    
1     98 48.762                                  
2     96 33.358  2    15.404 22.165 1.219e-08 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>