**Jan 18, 2018**

We will try the t test on the built-in data set `PlantGrowth`

. However, first we need to reformat the data set, which we do with the function `unstack()`

. We store the reformatted data set in a variable `plants`

:

`head(PlantGrowth)`

```
## weight group
## 1 4.17 ctrl
## 2 5.58 ctrl
## 3 5.18 ctrl
## 4 6.11 ctrl
## 5 4.50 ctrl
## 6 4.61 ctrl
```

```
plants <- unstack(PlantGrowth)
head(plants)
```

```
## ctrl trt1 trt2
## 1 4.17 4.81 6.31
## 2 5.58 4.17 5.12
## 3 5.18 4.41 5.54
## 4 6.11 3.59 5.50
## 5 4.50 5.87 5.37
## 6 4.61 3.83 5.29
```

The data set contains plant growth yield (dry weight) under one control and two treatment conditions:

`boxplot(plants)`

**Question:** Is the mean control weight significantly different from the mean weight under treatment 1? Is the mean weight under treatment 1 significantly different from the mean weight under treatment 2? Use the function `t.test()`

to find out.

`# R code goes here.`

We will try the correlation test on the built-in data set `cars`

. The data set contains the speed of cars and the distances taken to stop, measured in the 1920s:

`head(cars)`

```
## speed dist
## 1 4 2
## 2 4 10
## 3 7 4
## 4 7 22
## 5 8 16
## 6 9 10
```

Is there a relationship between speed and stopping distance? Use the function `cor.test()`

to find out. Then make a scatterplot of speed vs. stopping distance, using the function `plot()`

.

`# R code goes here.`

We will do a regression analysis on the data set `cabbages`

from the R package MASS. The data set contains the weight (`HeadWt`

), vitamin C content (`VitC`

), the cultivar (`Cult`

), and the planting date (`Date`

) for 60 cabbage heads:

```
library(MASS) # load the MASS library to make the data set available
head(cabbages)
```

```
## Cult Date HeadWt VitC
## 1 c39 d16 2.5 51
## 2 c39 d16 2.2 55
## 3 c39 d16 3.1 45
## 4 c39 d16 4.3 42
## 5 c39 d16 2.5 53
## 6 c39 d16 4.3 50
```

Use a multivariate regression to find out whether weight and cultivar have an effect on the vitamin C content. You will need to use the functions `lm()`

and `summary()`

.

`# R code goes here.`

Look into the function `predict()`

. Can you use it to estimate the vitamin C content of a c52 cultivar with a weight of 4? Can you use it to calculate the residuals of the regression model?

`# R code goes here.`