# QQ-plots: Quantile-Quantile plots - R Base Graphs

Previously, we described the essentials of R programming and provided quick start guides for importing data into **R**.

**quantile-quantile**plots in R.

**QQ plot**(or quantile-quantile plot) draws the correlation between a given sample and the normal distribution. A 45-degree reference line is also plotted. QQ plots are used to visually check the normality of the data.

# Pleleminary tasks

**Launch RStudio**as described here: Running RStudio and setting up your working directory**Prepare your data**as described here: Best practices for preparing your data and save it in an external .txt tab or .csv files**Import your data**into**R**as described here: Fast reading of data from txt|csv files into R: readr package.

# Example data

Here, we’ll use the built-in R data set named ToothGrowth.

```
# Store the data in the variable my_data
my_data <- ToothGrowth
```

# Create QQ plots

The R base functions **qqnorm**() and **qqplot**() can be used to produce quantile-quantile plots:

**qqnorm**(): produces a normal QQ plot of the variable**qqline**(): adds a reference line

```
qqnorm(my_data$len, pch = 1, frame = FALSE)
qqline(my_data$len, col = "steelblue", lwd = 2)
```

It’s also possible to use the function **qqPlot**() [in **car** package]:

```
library("car")
qqPlot(my_data$len)
```

As all the points fall approximately along this reference line, we can assume normality.

# See also

# Infos

This analysis has been performed using **R statistical software** (ver. 3.2.4).

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