Many of statistical tests including correlation, regression, t-test, and analysis of variance (ANOVA) assume some certain characteristics about the data. They require the data to follow a **normal distribution** or **Gaussian distribution**. These tests are called **parametric tests**, because their validity depends on the distribution of the data.

Normality and the other assumptions made by these tests should be taken seriously to draw reliable interpretation and conclusions of the research.

Before using a parametric test, we should perform some **preleminary tests** to make sure that the test assumptions are met. In the situations where the assumptions are violated, **non-paramatric** tests are recommended.

Here, we’ll describe how to check the normality of the data by visual inspection and by significance tests.

**dplyr**for data manipulation

`install.packages("dplyr")`

**ggpubr**for an easy ggplot2-based data visualization

- Install the latest version from GitHub as follow:

```
# Install
if(!require(devtools)) install.packages("devtools")
devtools::install_github("kassambara/ggpubr")
```

- Or, install from CRAN as follow:

`install.packages("ggpubr")`

```
library("dplyr")
library("ggpubr")
```

**Prepare your data**as specified here: Best practices for preparing your data set for R**Save your data**in an external .txt tab or .csv files**Import your data into R**as follow:

```
# If .txt tab file, use this
my_data <- read.delim(file.choose())
# Or, if .csv file, use this
my_data <- read.csv(file.choose())
```

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

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

We start by displaying a random sample of 10 rows using the function **sample_n**()[in **dplyr** package].

Show 10 random rows:

```
set.seed(1234)
dplyr::sample_n(my_data, 10)
```

```
len supp dose
7 11.2 VC 0.5
37 8.2 OJ 0.5
36 10.0 OJ 0.5
58 27.3 OJ 2.0
49 14.5 OJ 1.0
57 26.4 OJ 2.0
1 4.2 VC 0.5
13 15.2 VC 1.0
35 14.5 OJ 0.5
27 26.7 VC 2.0
```

We want to test if the variable *len* (tooth length) is normally distributed.

If the sample size is large enough (n > 30), we can ignore the distribution of the data and use parametric tests.

**The central limit theorem** tells us that no matter what distribution things have, the sampling distribution tends to be normal if the sample is large enough (n > 30).

However, to be consistent, normality can be checked by visual inspection [**normal plots (histogram)**, **Q-Q plot** (quantile-quantile plot)] or by **significance tests**].

**Density plot** and **Q-Q plot** can be used to check normality visually.

**Density plot**: the**density**plot provides a visual judgment about whether the distribution is bell shaped.

```
library("ggpubr")
ggdensity(my_data$len,
main = "Density plot of tooth length",
xlab = "Tooth length")
```

**Q-Q plot**:**Q-Q plot**(or quantile-quantile plot) draws the correlation between a given sample and the normal distribution. A 45-degree reference line is also plotted.

```
library(ggpubr)
ggqqplot(my_data$len)
```

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.

Visual inspection, described in the previous section, is usually unreliable. It’s possible to use a **significance test** comparing the sample distribution to a normal one in order to ascertain whether data show or not a serious deviation from normality.

There are several methods for **normality test** such as **Kolmogorov-Smirnov (K-S) normality test** and **Shapiro-Wilk’s test**.

The null hypothesis of these tests is that “sample distribution is normal”. If the test is **significant**, the distribution is non-normal.

**Shapiro-Wilk’s method** is widely recommended for normality test and it provides better power than K-S. It is based on the correlation between the data and the corresponding normal scores.

Note that, normality test is sensitive to sample size. Small samples most often pass normality tests. Therefore, it’s important to combine visual inspection and significance test in order to take the right decision.

The R function **shapiro.test**() can be used to perform the Shapiro-Wilk test of normality for one variable (univariate):

`shapiro.test(my_data$len)`

```
Shapiro-Wilk normality test
data: my_data$len
W = 0.96743, p-value = 0.1091
```

From the output, the p-value > 0.05 implying that the distribution of the data are not significantly different from normal distribution. In other words, we can assume the normality.

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

Here we’ll describe research questions and the corresponding statistical tests, as well as, the test assumptions.

The most popular research questions include:

- whether
**two variables**(n = 2) are**correlated**(i.e., associated) - whether
**multiple variables**(n > 2) are**correlated** - whether
**two groups**(n = 2) of samples**differ**from each other - whether
**multiple groups**(n >= 2) of samples**differ**from each other - whether the
**variability**of two samples differ

Each of these questions can be answered using the following statistical tests:

**Correlation test**between two variables**Correlation matrix**between multiple variables**Comparing the means of two groups**:**Student’s t-test**(parametric)**Wilcoxon rank test**(non-parametric)

**Comparing the means of more than two groups****ANOVA test**(analysis of variance, parametric): extension of t-test to compare more than two groups.**Kruskal-Wallis rank sum test**(non-parametric): extension of Wilcoxon rank test to compare more than two groups

**Comparing the variances**:- Comparing the variances of two groups:
**F-test**(parametric) - Comparison of the variances of more than two groups:
**Bartlett’s test**(parametric),**Levene’s test**(parametric) and**Fligner-Killeen test**(non-parametric)

- Comparing the variances of two groups:

Many of the statistical procedures including correlation, regression, t-test, and analysis of variance assume some certain characteristic about the data. Generally they assume that:

- the data are
**normally distributed** - and the
**variances**of the groups to be compared are**homogeneous**(equal).

These assumptions should be taken seriously to draw reliable interpretation and conclusions of the research.

These tests - correlation, t-test and ANOVA - are called **parametric tests**, because their validity depends on the distribution of the data.

Before using parametric test, we should perform some **preleminary tests** to make sure that the test assumptions are met. In the situations where the assumptions are violated, **non-paramatric** tests are recommended.

With

**large enough sample sizes**(n > 30) the violation of the normality assumption should not cause major problems (central limit theorem). This implies that we can ignore the distribution of the data and use parametric tests.However, to be consistent, we can use

**Shapiro-Wilk’s significance test**comparing the sample distribution to a normal one in order to ascertain whether data show or not a serious deviation from normality.

The standard **Student’s t-test** (comparing two independent samples) and the ANOVA test (comparing multiple samples) assume also that the samples to be compared have equal variances.

If the samples, being compared, follow normal distribution, then it’s possible to use:

**F-test**to compare the variances of two samples**Bartlett’s Test**or**Levene’s Test**to compare the variances of multiple samples.

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

- Import your data into R
- Check your data
- R functions for computing descriptive statistics
- Descriptive statistics for a single group
- Descriptive statistics by groups
- Frequency tables
- Infos

**Prepare your data**as specified here: Best practices for preparing your data set for R**Save your data**in an external .txt tab or .csv files**Import your data into R**as follow:

```
# If .txt tab file, use this
my_data <- read.delim(file.choose())
# Or, if .csv file, use this
my_data <- read.csv(file.choose())
```

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

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

You can inspect your data using the functions **head**() and **tails**(), which will display the first and the last part of the data, respectively.

```
# Print the first 6 rows
head(my_data, 6)
```

```
Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1 5.1 3.5 1.4 0.2 setosa
2 4.9 3.0 1.4 0.2 setosa
3 4.7 3.2 1.3 0.2 setosa
4 4.6 3.1 1.5 0.2 setosa
5 5.0 3.6 1.4 0.2 setosa
6 5.4 3.9 1.7 0.4 setosa
```

Some R functions for computing descriptive statistics:

Description | R function |
---|---|

Mean |
mean() |

Standard deviation |
sd() |

Variance |
var() |

Minimum |
min() |

Maximum |
maximum() |

Median |
median() |

Range of values (minimum and maximum) |
range() |

Sample quantiles |
quantile() |

Generic function |
summary() |

Interquartile range |
IQR() |

The function **mfv**(), for most frequent value, [in **modeest** package] can be used to find the statistical mode of a numeric vector.

Roughly speaking, the central tendency measures the “average” or the “middle” of your data. The most commonly used measures include:

- the mean: the average value. It’s sensitive to outliers.
- the median: the middle value. It’s a robust alternative to mean.
- and the mode: the most frequent value

In R,

- The function
**mean**() and**median**() can be used to compute the mean and the median, respectively; - The function
**mfv**() [in the**modeest**R package] can be used to compute the mode of a variable.

The R code below computes the mean, median and the mode of the variable *Sepal.Length* [in *my_data* data set]:

```
# Compute the mean value
mean(my_data$Sepal.Length)
```

`[1] 5.843333`

```
# Compute the median value
median(my_data$Sepal.Length)
```

`[1] 5.8`

```
# Compute the mode
# install.packages("modeest")
require(modeest)
mfv(my_data$Sepal.Length)
```

`[1] 5`

Measures of variability gives how “spread out” the data are.

**Range**corresponds to biggest value minus the smallest value. It gives you the full spread of the data.

```
# Compute the minimum value
min(my_data$Sepal.Length)
```

`[1] 4.3`

```
# Compute the maximum value
max(my_data$Sepal.Length)
```

`[1] 7.9`

```
# Range
range(my_data$Sepal.Length)
```

`[1] 4.3 7.9`

Recall that, quartiles divide the data into 4 parts. Note that, the **interquartile range** (IQR) - corresponding to the difference between the first and third quartiles - is sometimes used as a robust alternative to the standard deviation.

- R function:

`quantile(x, probs = seq(0, 1, 0.25))`

**x**: numeric vector whose sample quantiles are wanted.**probs**: numeric vector of probabilities with values in [0,1].

- Example:

`quantile(my_data$Sepal.Length)`

```
0% 25% 50% 75% 100%
4.3 5.1 5.8 6.4 7.9
```

By default, the function returns the minimum, the maximum and three **quartiles** (the 0.25, 0.50 and 0.75 quartiles).

To compute deciles (0.1, 0.2, 0.3, …., 0.9), use this:

`quantile(my_data$Sepal.Length, seq(0, 1, 0.1))`

To compute the interquartile range, type this:

`IQR(my_data$Sepal.Length)`

`[1] 1.3`

The variance represents the average squared deviation from the mean. The standard deviation is the square root of the variance. It measures the average deviation of the values, in the data, from the mean value.

```
# Compute the variance
var(my_data$Sepal.Length)
# Compute the standard deviation =
# square root of th variance
sd(my_data$Sepal.Length)
```

The median absolute deviation (MAD) measures the deviation of the values, in the data, from the median value.

```
# Compute the median
median(my_data$Sepal.Length)
# Compute the median absolute deviation
mad(my_data$Sepal.Length)
```

**Range**. It’s not often used because it’s very sensitive to outliers.**Interquartile range**. It’s pretty robust to outliers. It’s used a lot in combination with the median.**Variance**. It’s completely uninterpretable because it doesn’t use the same units as the data. It’s almost never used except as a mathematical tool**Standard deviation**. This is the square root of the variance. It’s expressed in the same units as the data. The standard deviation is often used in the situation where the mean is the measure of central tendency.**Median absolute deviation**. It’s a robust way to estimate the standard deviation, for data with outliers. It’s not used very often.

In summary, the IQR and the standard deviation are the two most common measures used to report the variability of the data.

The function **summary**() can be used to display several statistic summaries of either one variable or an entire data frame.

**Summary of a single variable**. Five values are returned: the mean, median, 25th and 75th quartiles, min and max in one single line call:

`summary(my_data$Sepal.Length)`

```
Min. 1st Qu. Median Mean 3rd Qu. Max.
4.300 5.100 5.800 5.843 6.400 7.900
```

**Summary of a data frame**. In this case, the function**summary**() is automatically applied to each column. The format of the result depends on the type of the data contained in the column. For example:- If the column is a numeric variable, mean, median, min, max and quartiles are returned.
- If the column is a factor variable, the number of observations in each group is returned.

`summary(my_data, digits = 1)`

```
Sepal.Length Sepal.Width Petal.Length Petal.Width Species
Min. :4 Min. :2 Min. :1 Min. :0.1 setosa :50
1st Qu.:5 1st Qu.:3 1st Qu.:2 1st Qu.:0.3 versicolor:50
Median :6 Median :3 Median :4 Median :1.3 virginica :50
Mean :6 Mean :3 Mean :4 Mean :1.2
3rd Qu.:6 3rd Qu.:3 3rd Qu.:5 3rd Qu.:1.8
Max. :8 Max. :4 Max. :7 Max. :2.5
```

It’s also possible to use the function **sapply**() to apply a particular function over a list or vector. For instance, we can use it, to compute for each column in a data frame, the mean, sd, var, min, quantile, …

```
# Compute the mean of each column
sapply(my_data[, -5], mean)
```

```
Sepal.Length Sepal.Width Petal.Length Petal.Width
5.843333 3.057333 3.758000 1.199333
```

```
# Compute quartiles
sapply(my_data[, -5], quantile)
```

```
Sepal.Length Sepal.Width Petal.Length Petal.Width
0% 4.3 2.0 1.00 0.1
25% 5.1 2.8 1.60 0.3
50% 5.8 3.0 4.35 1.3
75% 6.4 3.3 5.10 1.8
100% 7.9 4.4 6.90 2.5
```

The function **stat.desc**() [in **pastecs** package], provides other useful statistics including:

- the median
- the mean
- the standard error on the mean (SE.mean)
- the confidence interval of the mean (CI.mean) at the p level (default is 0.95)
- the variance (var)
- the standard deviation (std.dev)
and the variation coefficient (coef.var) defined as the standard deviation divided by the mean

Install

**pastecs**package

`install.packages("pastecs")`

- Use the function
**stat.desc**() to compute descriptive statistics

```
# Compute descriptive statistics
library(pastecs)
res <- stat.desc(my_data[, -5])
round(res, 2)
```

```
Sepal.Length Sepal.Width Petal.Length Petal.Width
nbr.val 150.00 150.00 150.00 150.00
nbr.null 0.00 0.00 0.00 0.00
nbr.na 0.00 0.00 0.00 0.00
min 4.30 2.00 1.00 0.10
max 7.90 4.40 6.90 2.50
range 3.60 2.40 5.90 2.40
sum 876.50 458.60 563.70 179.90
median 5.80 3.00 4.35 1.30
mean 5.84 3.06 3.76 1.20
SE.mean 0.07 0.04 0.14 0.06
CI.mean.0.95 0.13 0.07 0.28 0.12
var 0.69 0.19 3.12 0.58
std.dev 0.83 0.44 1.77 0.76
coef.var 0.14 0.14 0.47 0.64
```

Note that, when the data contains missing values, some R functions will return errors or NA even if just a single value is missing.

For example, the **mean()** function will return NA if even only one value is missing in a vector. This can be avoided using the argument **na.rm = TRUE**, which tells to the function to remove any NAs before calculations. An example using the **mean** function is as follow:

`mean(my_data$Sepal.Length, na.rm = TRUE)`

The R package **ggpubr** will be used to create graphs.

- Install the latest version from GitHub as follow:

```
# Install
if(!require(devtools)) install.packages("devtools")
devtools::install_github("kassambara/ggpubr")
```

- Or, install from CRAN as follow:

`install.packages("ggpubr")`

- Load ggpubr as follow:

`library(ggpubr)`

`ggboxplot(my_data, y = "Sepal.Length", width = 0.5)`

Histograms show the number of observations that fall within specified divisions (i.e., bins).

Histogram plot of Sepal.Length with mean line (dashed line).

```
gghistogram(my_data, x = "Sepal.Length", bins = 9,
add = "mean")
```

ECDF is the fraction of data smaller than or equal to x.

`ggecdf(my_data, x = "Sepal.Length")`

QQ plots is used to check whether the data is normally distributed.

`ggqqplot(my_data, x = "Sepal.Length")`

To compute summary statistics by groups, the functions **group_by**() and **summarise**() [in **dplyr** package] can be used.

- We want to group the data by
*Species*and then:- compute the number of element in each group. R function:
**n**() - compute the mean. R function
**mean**() - and the standard deviation. R function
**sd**()

- compute the number of element in each group. R function:

The function **%>%** is used to chaine operations.

- Install
**ddplyr**as follow:

`install.packages("dplyr")`

- Descriptive statistics by groups:

```
library(dplyr)
group_by(my_data, Species) %>%
summarise(
count = n(),
mean = mean(Sepal.Length, na.rm = TRUE),
sd = sd(Sepal.Length, na.rm = TRUE)
)
```

```
Source: local data frame [3 x 4]
Species count mean sd
(fctr) (int) (dbl) (dbl)
1 setosa 50 5.006 0.3524897
2 versicolor 50 5.936 0.5161711
3 virginica 50 6.588 0.6358796
```

- Graphics for grouped data:

```
library("ggpubr")
# Box plot colored by groups: Species
ggboxplot(my_data, x = "Species", y = "Sepal.Length",
color = "Species",
palette = c("#00AFBB", "#E7B800", "#FC4E07"))
```

```
# Stripchart colored by groups: Species
ggstripchart(my_data, x = "Species", y = "Sepal.Length",
color = "Species",
palette = c("#00AFBB", "#E7B800", "#FC4E07"),
add = "mean_sd")
```

Note that, when the number of observations per groups is small, it’s recommended to use **strip chart** compared to box plots.

A frequency table (or contingency table) is used to describe categorical variables. It contains the counts at each combination of factor levels.

R function to generate tables: **table**()

Distribution of hair and eye color by sex of 592 students:

```
# Hair/eye color data
df <- as.data.frame(HairEyeColor)
hair_eye_col <- df[rep(row.names(df), df$Freq), 1:3]
rownames(hair_eye_col) <- 1:nrow(hair_eye_col)
head(hair_eye_col)
```

```
Hair Eye Sex
1 Black Brown Male
2 Black Brown Male
3 Black Brown Male
4 Black Brown Male
5 Black Brown Male
6 Black Brown Male
```

```
# hair/eye variables
Hair <- hair_eye_col$Hair
Eye <- hair_eye_col$Eye
```

- Table of counts

```
# Frequency distribution of hair color
table(Hair)
```

```
Hair
Black Brown Red Blond
108 286 71 127
```

```
# Frequency distribution of eye color
table(Eye)
```

```
Eye
Brown Blue Hazel Green
220 215 93 64
```

- Graphics: to create the graphics, we start by converting the table as a data frame.

```
# Compute table and convert as data frame
df <- as.data.frame(table(Hair))
df
```

```
Hair Freq
1 Black 108
2 Brown 286
3 Red 71
4 Blond 127
```

```
# Visualize using bar plot
library(ggpubr)
ggbarplot(df, x = "Hair", y = "Freq")
```

```
tbl2 <- table(Hair , Eye)
tbl2
```

```
Eye
Hair Brown Blue Hazel Green
Black 68 20 15 5
Brown 119 84 54 29
Red 26 17 14 14
Blond 7 94 10 16
```

It’s also possible to use the function **xtabs**(), which will create cross tabulation of data frames with a formula interface.

`xtabs(~ Hair + Eye, data = hair_eye_col)`

- Graphics: to create the graphics, we start by converting the table as a data frame.

```
df <- as.data.frame(tbl2)
head(df)
```

```
Hair Eye Freq
1 Black Brown 68
2 Brown Brown 119
3 Red Brown 26
4 Blond Brown 7
5 Black Blue 20
6 Brown Blue 84
```

```
# Visualize using bar plot
library(ggpubr)
ggbarplot(df, x = "Hair", y = "Freq",
color = "Eye",
palette = c("brown", "blue", "gold", "green"))
```

```
# position dodge
ggbarplot(df, x = "Hair", y = "Freq",
color = "Eye", position = position_dodge(),
palette = c("brown", "blue", "gold", "green"))
```

- Hair and Eye color distributions by sex using
**xtabs**():

`xtabs(~Hair + Eye + Sex, data = hair_eye_col)`

```
, , Sex = Male
Eye
Hair Brown Blue Hazel Green
Black 32 11 10 3
Brown 53 50 25 15
Red 10 10 7 7
Blond 3 30 5 8
, , Sex = Female
Eye
Hair Brown Blue Hazel Green
Black 36 9 5 2
Brown 66 34 29 14
Red 16 7 7 7
Blond 4 64 5 8
```

- You can also use the function
**ftable**() [for flat contingency tables]. It returns a nice output compared to xtabs() when you have more than two variables:

`ftable(Sex + Hair ~ Eye, data = hair_eye_col)`

```
Sex Male Female
Hair Black Brown Red Blond Black Brown Red Blond
Eye
Brown 32 53 10 3 36 66 16 4
Blue 11 50 10 30 9 34 7 64
Hazel 10 25 7 5 5 29 7 5
Green 3 15 7 8 2 14 7 8
```

**Table margins** correspond to the sums of counts along rows or columns of the table. **Relative frequencies** express table entries as proportions of table margins (i.e., row or column totals).

The function **margin.table**() and **prop.table**() can be used to compute table margins and relative frequencies, respectively.

**Format of the functions**:

```
margin.table(x, margin = NULL)
prop.table(x, margin = NULL)
```

**x**: table**margin**: index number (1 for rows and 2 for columns)

**compute table margins**:

```
Hair <- hair_eye_col$Hair
Eye <- hair_eye_col$Eye
# Hair/Eye color table
he.tbl <- table(Hair, Eye)
he.tbl
```

```
Eye
Hair Brown Blue Hazel Green
Black 68 20 15 5
Brown 119 84 54 29
Red 26 17 14 14
Blond 7 94 10 16
```

```
# Margin of rows
margin.table(he.tbl, 1)
```

```
Hair
Black Brown Red Blond
108 286 71 127
```

```
# Margin of columns
margin.table(he.tbl, 2)
```

```
Eye
Brown Blue Hazel Green
220 215 93 64
```

**Compute relative frequencies**:

```
# Frequencies relative to row total
prop.table(he.tbl, 1)
```

```
Eye
Hair Brown Blue Hazel Green
Black 0.62962963 0.18518519 0.13888889 0.04629630
Brown 0.41608392 0.29370629 0.18881119 0.10139860
Red 0.36619718 0.23943662 0.19718310 0.19718310
Blond 0.05511811 0.74015748 0.07874016 0.12598425
```

```
# Table of percentages
round(prop.table(he.tbl, 1), 2)*100
```

```
Eye
Hair Brown Blue Hazel Green
Black 63 19 14 5
Brown 42 29 19 10
Red 37 24 20 20
Blond 6 74 8 13
```

To express the frequencies relative to the grand total, use this:

`he.tbl/sum(he.tbl)`

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