The function `bwplot()`

makes box-and-whisker plots for numerical variables. It comes from the `lattice`

package for statistical graphics, which is pre-installed with every distribution of R. Also, package `tigerstats`

depends on lattice, so if you load `tigerstats`

:

`require(tigerstats)`

then `lattice`

will be loaded as well.

For a box-and-whisker plot of the fastest speeds ever driven by students in the `m111survey`

data frame, use the command:

```
bwplot(~fastest,data=m111survey,
xlab="speed (mph)",
main="Fastest Speed Ever Driven")
```

Note the use of:

- the
`xlab`

argument to label the horizontal axis, complete with units (miles per hour); - the
`main`

argument to provide a brief but descriptive title for the graph.

Say you want to know:

Who tends to drive faster: the guys or the gals?

Then you are studying the relationship between the numerical variable **fastest** and the factor variable **sex**. `bwplot()`

will break the fastest speeds up by sex and parallel box-and-whisker plots, if you run the following command:

```
bwplot(fastest~sex,data=m111survey,
ylab="speed (mph)",
main="Fastest Speed Ever Driven,\nby Sex of Subject")
```

Note the use of the “” to create a two-line title. This trick can come in handy if your title is long!

If you prefer your box-and-whisker plots to be horizontal, then you can reverse the order of the variables in the formula:

```
bwplot(sex~fastest,data=m111survey,
xlab="speed (mph)",
main="Fastest Speed Ever Driven,\nby Sex of Subject")
```

Box-and-Whisker plots are great for studying:

- one numerical variable
- the relationship between a numerical variable and a factor variable.

However, if you try to study the relationship between two numerical variables with `bwplot()`

, you will get bizarre results.

For example, suppose you want to study the relationship between fastest speed ever driven (**fastest**) and the grade point average (**GPA**) of the subjects in `m111survey`

:

```
bwplot(fastest~GPA,data=m111survey,
ylab="speed (mph)",
xbal="grade-point average",
main="Fastes Speed Ever Driven,\nby Grade-Point Average")
```

`bwplot()`

expects at least one of the two variable in the formula to be numerical. When it is presented with two numerical variables it politely makes do—apparently converting **fastest** into a new factor variable—but the resulting graph doesn’t make any sense at all.

Use `xyplot()`

(scatterplots) to study the relationship between two numerical variables.