Rstudio

• Make a folder on your computer where you want to keep files to use in Rstudio. Do NOT use Danish characters æ, ø, å in the folder name (or anywhere in the path to the folder).
• Set the working directory to this folder: Session -> Set Working Directory -> Choose Directory (shortcut: Ctrl+Shift+H).
• Make the change permanent by setting the default directory in: Tools -> Global Options -> Choose Directory.

R basics

• Ordinary calculations:

4.6 * (2 + 3)^4 

## [1] 2875

• Make a (scalar) object and print it:

a <- 4 
a

## [1] 4

• Make a (vector) object and print it:

b <- c(2, 5, 7)
b

## [1] 2 5 7

• Make a sequence of numbers and print it:

s <- 1:4
s

## [1] 1 2 3 4

• Note: A more flexible command for sequences:

s <- seq(1, 4, by = 1)

• R does elementwise calculations:

a * b

## [1]  8 20 28

a + b

## [1]  6  9 11

b ^ 2

## [1]  4 25 49

• Sum and product of elements:

sum(b)

## [1] 14

prod(b)

## [1] 70

R markdown

• The slides and all exercises in R (including the exam questions) are made in the special Rmarkdown format.

• This allows you to combine text and R code.

• You can write formulas using standard LaTeX commands.

R extensions

• The functionality of R can be extended through libraries or packages (much like plugins in browsers etc.). Some are installed by default in R and you just need to load them.
• To install a new package in Rstudio use the menu: Tools -> Install Packages
• You need to know the name of the package you want to install. You can also do it through a command:

install.packages("mosaic")

• When it is installed you can load it through the library command:

library(mosaic)

• This loads the mosaic package which has a lot of convenient functions for this course (we will get back to that later). It also prints a lot of info about functions that have been changed by the mosaic package, but you can safely ignore that.

R help

• You get help via ?<command>:

?sum

• Use tab to make Rstudio guess what you have started typing.
• Search for help:

help.search("plot")

• You can find a cheat sheet with the R functions we use for this course here.
• Save your commands in a file for later usage:
  • Select history tab in top right pane in Rstudio .
  • Mark the commands you want to save.
  • Press To Source button.

Data example

• Now we will have a look at a data set concerning the 1988 vote in Chile for or against Pinochet to continue as leader. The sample consists of 2700 voters randomly selected from the Chilenean population.

• The data set contains the variables:
• region: The region in Chile where the voter lives
• population: Population of the region.
• sex: The gender of the voter.
• age: The age of the voter.
• education: Education level of the voter.
• income: Monthly income of the voter.
• statusquo: To which degree the voter supports the status quo.

• vote: Should Pinochet continue? Y = yes, N= no, U=undecided, A= will abstain from voting.

• More information about the data set may be found here.

Chile <- read.delim("https://asta.math.aau.dk/datasets?file=Chile.txt")
head(Chile)

##   region population sex age education income statusquo vote
## 1      N     175000   M  65         P  35000       1.0    Y
## 2      N     175000   M  29        PS   7500      -1.3    N
## 3      N     175000   F  38         P  15000       1.2    Y
## 4      N     175000   F  49         P  35000      -1.0    N
## 5      N     175000   F  23         S  35000      -1.1    N
## 6      N     175000   F  28         P   7500      -1.0    N

Data types

Quantitative variables

• The measurements have numerical values.
• Quantative data often comes about in one of the following ways:
  • Continuous variables: measurements of e.g. speed, temperature, etc.
  • Discrete variables: counts of e.g. number of household members, hits on a webpage, cars passing on a road in one hour, etc.
• Measurements like this have a well-defined scale and in R they are stored as the type numeric.
• It is important to be able to distinguish between discrete count variables and continuous variables, since this often determines how we describe the uncertainty of a measurement.

Categorical/qualitative variables

• The measurement is one of a set of given categories, e.g. sex (male/female), education level, satisfaction score (low/medium/high), etc.
• Factors have two so-called scales:
  • Nominal scale: There is no natural ordering of the factor levels, e.g. sex and hair color.
  • Ordinal scale: There is a natural ordering of the factor levels, e.g. education level and satisfaction score.
• The measurement is usually stored (which is also recommended) as a factor in R. The possible categories are called levels. Example: the levels of the factor “sex” is male/female. A factor in R can have a so-called attribute assigned, which tells if it is ordinal.

Variables in the data set

head(Chile)

##   region population sex age education income statusquo vote
## 1      N     175000   M  65         P  35000       1.0    Y
## 2      N     175000   M  29        PS   7500      -1.3    N
## 3      N     175000   F  38         P  15000       1.2    Y
## 4      N     175000   F  49         P  35000      -1.0    N
## 5      N     175000   F  23         S  35000      -1.1    N
## 6      N     175000   F  28         P   7500      -1.0    N

• Quantitative variables in the Chile data set:

• population, age, income, statusquo

• Categorical variables:

• region, sex, education, vote

• All the categorical variables are nominal except education, which has three ordered categories (primary, secondary, post-secondary).

Tables

• To summarize the the variable vote we can use the function tally from the mosaic package (remember the package must be loaded via library(mosaic) if you did not do so yet):

tally( ~ vote, data = Chile)

## vote
##    A    N    U    Y <NA> 
##  187  889  588  868  168

• In percent:

tally( ~ vote, data = Chile, format = "percent")

## vote
##    A    N    U    Y <NA> 
##  6.9 32.9 21.8 32.1  6.2

• Here we use an R formula (characterized by the “tilde” sign ~) to indicate that we want this variable from the dataset Chile (without the tilde it would look for a global variable called vote and use that rather than the one in the dataset).

2 factors: Cross tabulation

• To get an overview over the relation between two categorical variables, we can make a cross tabulation.

• To make a table of all combinations of the two factors vote and sex, we use tally again:

tally( ~ vote + sex, data = Chile)

##       sex
## vote     F   M
##   A    104  83
##   N    363 526
##   U    362 226
##   Y    480 388
##   <NA>  70  98

• We can also get the relative frequencies (in percent) columnwise:

tally( ~ vote | sex, data = Chile, format = "percent")

##       sex
## vote      F    M
##   A     7.5  6.3
##   N    26.3 39.8
##   U    26.3 17.1
##   Y    34.8 29.4
##   <NA>  5.1  7.4

• For instance we see that \(34.8\%\) of the women said they would vote yes, while this holds for only \(29.4\%\) of the men.

Visualizing categorical data: Bar graph

• To create a bar graph plot of table data we use the function gf_bar from mosaic. For each level of the factor, a box is drawn with the height proportional to the frequency (count) of the level.

gf_bar( ~ vote, data = Chile)

• The bar graph can also be split by group:

gf_bar( ~ vote | sex, data = Chile)

Data example: Fuel consumption of cars

• In this data set, a car magazine tested the fuel consumption of 32 cars. The variable mpg gives the fuel consumption in miles pr. gallon (the data set is from 1974).

• The data set is built into R under the name mtcars, so it does not need to be loaded before use.

head(mtcars)

##                   mpg cyl disp  hp drat  wt qsec vs am gear carb
## Mazda RX4          21   6  160 110  3.9 2.6   16  0  1    4    4
## Mazda RX4 Wag      21   6  160 110  3.9 2.9   17  0  1    4    4
## Datsun 710         23   4  108  93  3.9 2.3   19  1  1    4    1
## Hornet 4 Drive     21   6  258 110  3.1 3.2   19  1  0    3    1
## Hornet Sportabout  19   8  360 175  3.1 3.4   17  0  0    3    2
## Valiant            18   6  225 105  2.8 3.5   20  1  0    3    1

Visualizing quantitative data: Histogram

• The way to get a first impression of a quantitative variable is to draw a histogram.

• The histogram of a variable x is made as follows:
  • Divide the interval from the minimum value of x to the maximum value of x in an appropriate number of equal sized sub-intervals.
  • Draw a box over each sub-interval with the height being proportional to the number of observations in the sub-interval.

• Histogram of mpg for the mtcars data. The bins option sets the number of subintervals to 10.

gf_histogram( ~ mpg, data = mtcars,bins=10)

Relation between histogram and denity function

• Suppose a sample comes from a population having a continuous distribution with density function \(f\).

• Draw a histogram where the \(y\)-axis is scaled such that the total area of the bars is 1.
• When the number of observations (the sample size) increases we can make a finer interval division and get a more smooth histogram.

• When the number of observations tends to infinity, we obtain a nice smooth curve, where the area below the curve is \(1\). This curve is exactly the probability density function \(f\).

• If the histogram looks bell-shaped this may suggest a normal distribution.

Summary statistics for quantitative data

• We return to the mtcars example. A summary of the fuel consumption mpg can be retrieved using the favstats function:

favstats( ~ mpg, data = mtcars)

##  min Q1 median Q3 max mean sd  n missing
##   10 15     19 23  34   20  6 32       0

• The output contains the following information
• min The minimal value in the sample is \(10.4\).
• max The maximal value in the sample is \(33.9\).
• n The sample size (number of observations) is 32.
• mean The sample mean is \(20.1\). Recall that this was the average of all observations \(x_1,\ldots,x_n\), i.e. \[ \bar{x}=\frac{1}{n}\sum_{i=1}^n x_i \]

• sd The sample standard deviation is \(6.03\). Recall that this was given by \[ s=\frac{1}{n-1}\sum_{i=1}^n (x_i-\bar{x})^2. \]

• missing There are no missing values.

• median The median (or 50-percentile) is the value such that half of the sample has lower values than the median and half the sample has larger values.

• Q1 and Q3 will be introduced on later slides.

• Both the mean and the median can be considered the center of a distribution. In a symmetric distribution (such as the normal distribution) they are equal, while in a skewed distribution, they tend to be different.

Calculation of mean, median and standard deviation using R

• The mean, median and standard deviation are just some of the summaries that can be read of the favstats output (shown on previous page). They may also be calculated separately in the following way:

• Sample size of mpg:

length(mtcars$mpg) 

## [1] 32

• Mean of mpg:

mean( ~ mpg, data = mtcars)

## [1] 20

• Median of mpg:

median( ~ mpg, data = mtcars)

## [1] 19

• Standard deviation for mpg:

sd( ~ mpg, data = mtcars)

## [1] 6

• We may also calculate the summaries within groups. For instance, for each engine type (variable vs) the sample mean is:

mean( ~ mpg | factor(vs), data = mtcars)

##  0  1 
## 17 25

Interpretation of summary statistics: The empirical rule

• If the histogram of the sample looks like a bell shaped curve, then we have

• about 68% of the observations lie between \(\bar{y}-s\) and \(\bar{y}+s\).
• about 95% of the observations lie between \(\bar{y}-2s\) and \(\bar{y}+2s\).

• All or almost all (99.7%) of the observations lie between \(\bar{y}-3s\) and \(\bar{y}+3s\).

Percentiles

• The \(p\)th percentile is a value such that about \(p\)% of the population (or sample) lies below or at this value and about \((100-p)\)% of the population (or sample) lies above it.

Percentile calculation for a sample:

• First, sort data from smallest to largest. For the mpg variable: \[ x_{(1)}=10.4, x_{(2)}=10.4, x_{(3)}=13.3, \ldots, x_{(n)} = 33.9. \] Here the number of observations is \(n=32\).

• Find the \(10\)th percentile (i. e. \(p = 10\)): 
  • The observation number corresponding to the 10-percentile is \(N = \frac{ 32 \cdot 10}{100} = 3.2\).
  • So the 10-percentile lies between the observations with observation number \(k=3\) and \(k+1=4\). That is, its value lies somewhere in the interval between \(x_{(3)}=13.3\) and \(x_{(4)}=14.3\).
  • One of several methods for estimating the 10-percentile from the value of N is defined as: \[ x_{(k)} + (N - k)(x_{(k+1)} - x_{(k)}) \] which in this case gives \[ x_{(3)} + (3.2 - 3)(x_{(4)} - x_{(3)}) = 13.3 + 0.2 \cdot (14.3-13.3) = 13.5. \]

Median, quartiles and interquartile range

Recall

favstats( ~ mpg, data = mtcars)

##  min Q1 median Q3 max mean sd  n missing
##   10 15     19 23  34   20  6 32       0

• 0-percentile = 10.4 is the minimum value.
• 50-percentile = 20.1 is the median and it is a measure of the center of data.
• 25-percentile = 15.4 is called the lower quartile (Q1). Median of lower 50% of data.
• 75-percentile = 22.8 is called the upper quartile (Q3). Median of upper 50% of data.
• 100-percentile = 33.9 is the maximum value.
• Interquartile Range (IQR): a measure of variability given by the difference of the upper and lower quartiles: 23 -15 = 8.

Box-and-whiskers plots (or simply box plots)

How to draw a box-and-whiskers plot:

• Box:
  • Calculate the median, lower and upper quartiles.
  • Plot a line by the median and draw a box between the upper and lower quartiles.
• Whiskers:
  • Calculate interquartile range and call it IQR.
  • Calculate the following values:
    • L = lower quartile - 1.5*IQR
    • U = upper quartile + 1.5*IQR
  • Draw a line from lower quartile to the smallest measurement, which is larger than L.
  • Similarly, draw a line from upper quartile to the largest measurement which is smaller than U.

• Outliers: Measurements smaller than L or larger than U are drawn as circles.

• Note: Whiskers are minimum and maximum of the observations that are not deemed to be outliers.

Boxplot for fuel consumption

• Boxplot of the fuel consumption separately for each engine type:

favstats(mpg ~ vs, data = mtcars)

##   vs min Q1 median Q3 max mean  sd  n missing
## 1  0  10 15     16 19  26   17 3.9 18       0
## 2  1  18 21     23 30  34   25 5.4 14       0

gf_boxplot(mpg ~ factor(vs), data = mtcars)

• Cars with engine type 1 seem to use more fuel.
• A single car with engine type 0 differs noticeably from the others with a high fuel consumption.

2 quantitative variables: Scatter plot

• A scatter plot is used to visualize two quantitative variables.

• For instance, we can plot the relation between fuel consumption and horse powers (hp) of a car as follows

gf_point(mpg ~ hp, data = mtcars)

• This can be either split or coloured according to the engine typevs:

gf_point(mpg ~ hp | factor(vs), data = mtcars)

gf_point(mpg ~ hp, col = ~factor(vs), data = mtcars)

• If we want a regression line along with the points we can do:

gf_point(mpg ~ hp, col = ~factor(vs), data = mtcars) %>% gf_lm()

The empirical quantiles

• Recall that the distribution function of a random variable \(X\) was defined as: \[F(x)=P(X\leq x).\]
• The \(\frac{i}{n}\) quantiles of a distribution are the points \(q_i\) such that \(F(q_i)=\frac{i}{n}\), \(i=1,\ldots,n\).
• If we rank the observations in a sample \[x_{(1)}\leq x_{(2)}\leq \ldots \leq x_{(n)},\] we can approximate \(F\) at \(x_{(i)}\) by: \[\hat{F}(x_{(i)}) = \frac{i}{n}.\]
• Interpretation: \(x_{(i)}\) is approximately the \(\frac{i}{n}\) quantile.
• Note: various authors may use slightly different quantiles, e.g. \(\frac{i-0.5}{n}\) quantiles.

Normal quantile-quantile plots

• The quantiles may be used to investigate whether the sample comes from a normal distribution.

• Call the \(\frac{i}{n}\)th quantile of a standard normal distribution \(q_i\), i.e. \(P(Z\leq q_i)= \frac{i}{n}\).

• If \(Y\sim \texttt{norm}(\mu,\sigma)\), then this is equivalent to is equivalent to \[P(Y\leq \mu+\sigma q_i)= \frac{i}{n}.\]

• Suppose the population follows a \(\texttt{norm}(\mu,\sigma)\) distribution, then the sample quantiles \(x_{(i)}\) should be approximately equal to the population quantiles \(\mu+\sigma q_i\).

• If we make a scatter plot of the pair \((q_i,x_{(i)})\), these should lie on a straight line. We call this a normal Q-Q plot (or quantile-quantile plot).

• Example: We investigate whether the mpg variable in the mtcars data set follows a normal distribution:

qqnorm(mtcars$mpg)
qqline(mtcars$mpg)