In this example, we will complete a linear regression in R using mtcars, one of the built-in R datasets.

1. help or ?

First, let us get acquainted with the mtcars dataset.

To do so, we look at the package description. We can do this by either using ‘?mtcars’ or ‘help(mtcars)’.

*Note: this will print in the Help window, and has been pasted in to the Notebook for completeness.*

`help(mtcars)`

mtcars {datasets}

R Documentation

Motor Trend Car Road Tests

DescriptionThe data was extracted from the 1974 Motor Trend US magazine, and comprises fuel consumption and 10 aspects of automobile design and performance for 32 automobiles (1973–74 models).

Usagemtcars

FormatA data frame with 32 observations on 11 variables.

[, 1] mpg Miles/(US) gallon

[, 2] cyl Number of cylinders

[, 3] disp Displacement (cu.in.)

[, 4] hp Gross horsepower

[, 5] drat Rear axle ratio

[, 6] wt Weight (1000 lbs)

[, 7] qsec 1/4 mile time

[, 8] vs V/S

[, 9] am Transmission (0 = automatic, 1 = manual)

[,10] gear Number of forward gears

[,11] carb Number of carburetors

SourceHenderson and Velleman (1981), Building multiple regression models interactively. Biometrics, 37, 391–411.

Examplesrequire(graphics)

pairs(mtcars, main = “mtcars data”)

coplot(mpg ~ disp | as.factor(cyl), data = mtcars,

panel = panel.smooth, rows = 1

2. head

Let’s look at the data itself.

*The head function returns the first six lines of the data frame. You can also specify the number of lines to display as the second argument to head. For example, head(mtcars, 10) would display the first 10 lines.*

`head(mtcars)`

mpg cyl disp hp drat wt qsec vs am gear carb Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4 Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4 Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1 Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1 Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2 Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1

3. dim

*The dim function gives us the dimensions of the dataset.*

`dim(mtcars)`

`[1] 32 11`

It looks like mtcars is a small dataset, containing information for 32 different car models.

4. summary (of data frame)

Let’s further familiarize ourselves with the data.

*The summary of a dataframe returns the quartiles of each numeric column of the dataset.*

`summary(mtcars)`

```
mpg cyl disp hp
Min. :10.40 Min. :4.000 Min. : 71.1 Min. : 52.0
1st Qu.:15.43 1st Qu.:4.000 1st Qu.:120.8 1st Qu.: 96.5
Median :19.20 Median :6.000 Median :196.3 Median :123.0
Mean :20.09 Mean :6.188 Mean :230.7 Mean :146.7
3rd Qu.:22.80 3rd Qu.:8.000 3rd Qu.:326.0 3rd Qu.:180.0
Max. :33.90 Max. :8.000 Max. :472.0 Max. :335.0
drat wt qsec vs
Min. :2.760 Min. :1.513 Min. :14.50 Min. :0.0000
1st Qu.:3.080 1st Qu.:2.581 1st Qu.:16.89 1st Qu.:0.0000
Median :3.695 Median :3.325 Median :17.71 Median :0.0000
Mean :3.597 Mean :3.217 Mean :17.85 Mean :0.4375
3rd Qu.:3.920 3rd Qu.:3.610 3rd Qu.:18.90 3rd Qu.:1.0000
Max. :4.930 Max. :5.424 Max. :22.90 Max. :1.0000
am gear carb
Min. :0.0000 Min. :3.000 Min. :1.000
1st Qu.:0.0000 1st Qu.:3.000 1st Qu.:2.000
Median :0.0000 Median :4.000 Median :2.000
Mean :0.4062 Mean :3.688 Mean :2.812
3rd Qu.:1.0000 3rd Qu.:4.000 3rd Qu.:4.000
Max. :1.0000 Max. :5.000 Max. :8.000
```

The following variables appear to be ordinal with a small subset of values, and could be treated as factors:

– cyl (number of cylinders): 4-8

– vs (V- or Standard engine): 0 or 1

– am (automatic or manual): 0 (automatic) or 1 (manual)

– gear (number of forward gears): 3-5

– carb (number fo carburators): 1-8

5. hist and boxplot

We can also look at boxplots and histograms of the variables.

*par(mfrow) divides up the plot area so several plots can be displayed simultaneously. Specifically, par(mfrow=c(2,2)) divides the display area into a 2 x 2 grid, to display 4 plots.*

```
par(mfrow=c(2,2))
hist(mtcars$mpg, main = "Histogram of MPG")
boxplot(mtcars$mpg, main = "Boxplot of MPG")
```

```
hist(mtcars$wt, main = "Histogram of Weight")
boxplot(mtcars$wt, main = "Boxplot of Weight")
```

6. boxplot (by factor)

Given that we have several variables that can be treated as factors, we may also want to look at conditional boxplots.

*The ‘~’ notation is R notation for ‘function of’, so boxplot(mpg ~ gear) gives a boxplot for each category in gear.*

```
par(mfrow=c(2,2))
boxplot(mpg ~ gear, data = mtcars, col = "light gray", main = "MPG by Gear")
boxplot(mpg ~ am, data = mtcars, col = "light gray", main = "MPG by A/M")
```

```
boxplot(mpg ~ vs, data = mtcars, col = "light gray", main = "MPG by V/S")
boxplot(mpg ~ carb, data = mtcars, col = "light gray", main = "MPG by Carb")
```

7. pairs

We examine the correlation between the variables, first visually.

* The pairs function plots pairwise scatterplots of the variables in the dataframe, helping to quickly see the relationship amongst the variables. The plot is a bit scrunched here in the Notebook, and therefore not as informative as it could be in larger-sized plot.*

`pairs(mtcars)`

8. corrplot

We could also create a fancier correlational plot using the corrplot library to visualize the corelation matrix.

* You may need to install the corrplot library first, using Install Packages from the R Studio Tools menu, or the install function. Load the library into the current session using the library function: library(corrplot).*

```
library(corrplot)
M <- cor(mtcars) ## get the corelation matrix
corrplot.mixed(M) ## visualize the corelation matrix
```

From the correlational plots, we can see that many of our variables (or Xs) are highly correlated. We can also see that wt is highly correlated with mpg, which fits well with what we intuitively know about cars – that the mpg we get will depend on the weight of the car.

9. plot (scatterplots)

Before doing the regression, we may want to look at the scatterplots of mpg with the independent variables in more detail.

* The text function adds labels, row numbers of the observations in this case. *

```
plot(mpg ~ wt, data = mtcars)
text(mtcars$wt, mtcars$mpg, labels=seq(1:32), cex= 0.7, pos = 4)
```

```
plot(mpg ~ disp, data = mtcars)
text(mtcars$disp, mtcars$mpg, labels=seq(1:32), cex= 0.7, pos = 4)
```

```
plot(wt ~ disp, data = mtcars)
text(mtcars$disp, mtcars$wt, labels=seq(1:32), cex= 0.7, pos = 4)
```

These three scatterplots confirm what we learned from the correlational plots – both wt and disp are highly correlated with mpg and with each other.

10. lm

After having familiarized ourselves with the data, we can begin answering questions that might interest us; for example: how does mpg vary with other car attributes?

To answer the question, we begin with the simplest regression model – one with a single predictor. We choose wt as the predictor as it is both highly correlated with mpg and we inutitively expect the weight of a car to have a direct impact on mpg.

*The R function lm – for linear model – is the function we use for linear regression. The summary function (on lm results) provides a summary of the regression model, including the co-efficients, p-values and R ^{2}. *

```
m1 <- lm(mpg ~ wt, data = mtcars)
summary(m1)
```

```
Call:
lm(formula = mpg ~ wt, data = mtcars)
Residuals:
Min 1Q Median 3Q Max
-4.5432 -2.3647 -0.1252 1.4096 6.8727
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 37.2851 1.8776 19.858 < 2e-16 ***
wt -5.3445 0.5591 -9.559 1.29e-10 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Residual standard error: 3.046 on 30 degrees of freedom
Multiple R-squared: 0.7528, Adjusted R-squared: 0.7446
F-statistic: 91.38 on 1 and 30 DF, p-value: 1.294e-10
```

Just the weight of the car, wt, explains about 74% of the variance in mpg in our sample of cars. We have seen from the variable scatterplots and the correlational plots that disp is highly correlated with wt. So the next variable to add would be either cyl or hp, both of which are also highly correlated with each other. Let’s begin with testing out cyl.

```
m2 <- lm(mpg ~ wt + cyl, data = mtcars)
summary(m2)
```

```
Call:
lm(formula = mpg ~ wt + cyl, data = mtcars)
Residuals:
Min 1Q Median 3Q Max
-4.2893 -1.5512 -0.4684 1.5743 6.1004
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 39.6863 1.7150 23.141 < 2e-16 ***
wt -3.1910 0.7569 -4.216 0.000222 ***
cyl -1.5078 0.4147 -3.636 0.001064 **
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Residual standard error: 2.568 on 29 degrees of freedom
Multiple R-squared: 0.8302, Adjusted R-squared: 0.8185
F-statistic: 70.91 on 2 and 29 DF, p-value: 6.809e-12
```

Together wt and cyl explain 82% of the variance in mpg – not bad. How does the wt + hp model compare?

```
m2a <- lm(mpg ~ wt + hp, data = mtcars)
summary(m2a)
```

```
Call:
lm(formula = mpg ~ wt + hp, data = mtcars)
Residuals:
Min 1Q Median 3Q Max
-3.941 -1.600 -0.182 1.050 5.854
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 37.22727 1.59879 23.285 < 2e-16 ***
wt -3.87783 0.63273 -6.129 1.12e-06 ***
hp -0.03177 0.00903 -3.519 0.00145 **
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Residual standard error: 2.593 on 29 degrees of freedom
Multiple R-squared: 0.8268, Adjusted R-squared: 0.8148
F-statistic: 69.21 on 2 and 29 DF, p-value: 9.109e-12
```

The two models are pretty comparable…the R^{2} for the two models are very close, and both predictors are significant…which model to choose?

We could also try adding a third variable to the model. Since there are only 32 samples in the dataset, 3 predictors is probably enough, following the rule of thumb of ~10 observations per predictor. Below, we add am…you can play with others. Since the predictor variables are highly correlated, the results will be similar.

```
m3 <- lm(mpg ~ wt + cyl + am, data = mtcars)
summary(m3)
```

```
Call:
lm(formula = mpg ~ wt + cyl + am, data = mtcars)
Residuals:
Min 1Q Median 3Q Max
-4.1735 -1.5340 -0.5386 1.5864 6.0812
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 39.4179 2.6415 14.923 7.42e-15 ***
wt -3.1251 0.9109 -3.431 0.00189 **
cyl -1.5102 0.4223 -3.576 0.00129 **
am 0.1765 1.3045 0.135 0.89334
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Residual standard error: 2.612 on 28 degrees of freedom
Multiple R-squared: 0.8303, Adjusted R-squared: 0.8122
F-statistic: 45.68 on 3 and 28 DF, p-value: 6.51e-11
```

We see from the summary for model m3 that am does not add any predictive power to the model – the R^{2} is essentially unchanged, and am is not significant (the p-value is 0.89) – if wt and cyl are already included in the model.

11. vif

Given the high degree of correlation – or multi-collinearity – of the predictor variables, it is probably a good idea to check the VIF (variance inflation factor).

* You may need to install the car (companion to applied regression) package, and load the library. *

```
library(car)
vif(m2); vif(m2a); vif(m3)
```

```
wt cyl
2.579312 2.579312
wt hp
1.766625 1.766625
wt cyl am
3.609011 2.584066 1.924955
```

Depending on the purpose of the model, VIFs of 10 or even higher are okay. The model VIFs which are all below 3.7 indicate that they do not have excessive multi-collinearity. However, since am does not add any predictive power, there is no need to consider m3 any further.

12. Diagnostics plots for lm

How do we choose between m2 (mpg ~ wt + cyl) and m2a (mpg ~ wt + hp)? One way is to see which model has better diagnostics.

* Note that diagnostics tell you how well the model fits the underlying assumptions of linear regression. So it is an important step for all regression models.*

```
par(mfrow=c(2,2))
plot(m2)
```

```
par(mfrow = c(2,2))
plot(m2a)
```

Comparing the diagnostics for m2 (mpg ~ wt + cyl) and m2a (mpg ~ wt + hp), we see that the diagnostics are a little bit better for m2a. The residual plots, which test the assumption that the error term (residuals) are random, are comparable – may be marginally better for m2a, while the qq plot, which tests the assumption of normality of the residuals,is slightly better than for m2a than m2. The scale-location plot, which is a test for residuals being independent of predictors, might be a little bit better for m2a, and the leverage plots (which measure the influence of individual observations on the model) are comparable. All in all, based on our sample set, m2a might be a slightly better model choice.

Next steps

R packages (stats, corrplot, car, MASS) make developing descriptive and predictive linear regression models relatively simple. We developed a regression model using a very small subset of R funtionality. For example, for a data set with a greater number of predictors and observations, we could do a stepwise regression using the step function in the stats package or stepAIC in MASS package. We can also fit polynomials and look for interactions between variables. For more complex statistical modeling, R has packages like gam (general additive models); glm (generalized linear models) and nlme (multi-level modeling).