Introduction to R and data exploration

Martijn Wieling
University of Groningen

This lecture

  • RStudio and R
  • R as calculator
  • Variables
  • Functions and help
  • Importing data in R in a dataframe
  • Accessing rows and columns
  • Adding columns to the data
  • Goal of statistics
  • Data exploration (descriptive statistics)
    • Numerical measures
    • Visual exploration

Our tool: RStudio (frontend to R)

RStudio: quick overview

Basic functionality: R as calculator

# Addition (this is a comment: preceded by '#')
5 + 5
# [1] 10
# Multiplication
5 * 3
# [1] 15
# Division
5/3
# [1] 1.6667

Basic functionality: using variables

a <- 5  # store a single value; instead of '<-' you can also use '='
a  # display the value
# [1] 5
b <- c(2, 4, 6, 7, 8)  # store a series of values in a vector
b
# [1] 2 4 6 7 8
b[4] <- a  # assign value 5 (stored in 'a') to the 4th element of vector b
b[1] <- NA  # assign NA (missing) to the first element of vector b
b <- b * 10  # multiply all values in vector b with 10
b
# [1] NA 40 60 50 80

Question 1

Basic functionality: using functions

mn <- mean(b)  # calculating the mean and storing in variable mn
mn
# [1] NA
# mn is NA (missing) as one of the values is missing
mean(b, na.rm = TRUE)  # we can use the function parameter na.rm to ignore NAs
# [1] 57.5
# But which parameters does a function have: use help!
help(mean)  # alternatively: ?mean

Basic functionality: a help file

Question 2

Try it yourself!

  • There are many resources for R which you can easily find online
  • Here we use "swirl" an online platform for interactive R courses
  • Start RStudio, install and start swirl:
install.packages("swirl", repos = "http://cran.rstudio.com/")
library(swirl)
swirl()
  • Follow the prompts and install the course R programming: The basics of programming in R
  • Choose that course to start with and finish Lesson 1 of that course

Getting data into R: exporting a data set

Getting data into R: importing a data set

setwd("C:/Users/Martijn/Desktop/Statistics/Intro-R")  # set working directory
dat <- read.csv2("thnl.csv")  # read.csv2 reads Excel csv file from work dir
str(dat)  # shows structure of the data frame dat (note: wide format)
# 'data.frame': 19 obs. of  4 variables:
#  $ Participant : Factor w/ 19 levels "VENI-NL_1","VENI-NL_10",..: 1 2 3 4 5 6 7 8 9 10 ...
#  $ Gender      : Factor w/ 2 levels "F","M": 2 2 2 2 2 2 1 2 2 1 ...
#  $ Frontness.T : num  0.781 0.766 0.884 0.748 0.748 ...
#  $ Frontness.TH: num  0.738 0.767 0.879 0.761 0.774 ...
dim(dat)  # number of rows and columns of data set
# [1] 19  4

Investigating imported data set: using head

head(dat)  # show first few rows of dat 
#   Participant Gender Frontness.T Frontness.TH
# 1   VENI-NL_1      M     0.78052      0.73801
# 2  VENI-NL_10      M     0.76621      0.76685
# 3  VENI-NL_11      M     0.88366      0.87871
# 4  VENI-NL_12      M     0.74757      0.76094
# 5  VENI-NL_13      M     0.74761      0.77420
# 6  VENI-NL_14      M     0.75186      0.74913

Question 3

Investigating imported data set: using RStudio viewer

Subsetting the data: indices and names

dat[1, ]  # values in first row
#   Participant Gender Frontness.T Frontness.TH
# 1   VENI-NL_1      M     0.78052      0.73801
dat[1:2, c(2, 3)]  # values of first two rows for second and third column
#   Gender Frontness.T
# 1      M     0.78052
# 2      M     0.76621
dat[c(1, 2, 3), "Participant"]  # values of first three rows for column 'Participant'
# [1] VENI-NL_1  VENI-NL_10 VENI-NL_11
# 19 Levels: VENI-NL_1 VENI-NL_10 VENI-NL_11 VENI-NL_12 VENI-NL_13 VENI-NL_14 ... VENI-NL_9
tmp <- dat[5:8, c(1, 3)]  # store columns 1 and 3 for rows 5 to 8 in tmp

Question 4

Subsetting the data: conditional indexing

tmp <- dat[dat$Gender == "M", ]  # only observations for male participants
head(tmp, n = 2)  # show first two rows
#   Participant Gender Frontness.T Frontness.TH
# 1   VENI-NL_1      M     0.78052      0.73801
# 2  VENI-NL_10      M     0.76621      0.76685
# more advanced subsetting: include rows for which frontness for the T sound is
# higher than 0.74 AND participant is either 1 or 2 N.B. use '|' instead of '&'
# for logical OR
dat[dat$Frontness.T > 0.74 & dat$Participant %in% c("VENI-NL_1", "VENI-NL_2"), ]
#   Participant Gender Frontness.T Frontness.TH
# 1   VENI-NL_1      M     0.78052      0.73801

Question 5

Supplementing the data: adding columns

# new column Diff containing difference between TH and T positions
dat$Diff <- dat$Frontness.TH - dat$Frontness.T

# new column DiffClass, initially all observations set to TH0
dat$DiffClass <- "TH0"

# observations with Diff larger than 0.02 are categorized as TH1, negative as TH-
dat[dat$Diff > 0.02, ]$DiffClass <- "TH1"
dat[dat$Diff < 0, ]$DiffClass <- "TH-"

dat$DiffClass <- factor(dat$DiffClass)  # convert string variable to factor

head(dat, 2)
#   Participant Gender Frontness.T Frontness.TH        Diff DiffClass
# 1   VENI-NL_1      M     0.78052      0.73801 -0.04250668       TH-
# 2  VENI-NL_10      M     0.76621      0.76685  0.00064245       TH0

Question 6

Try it yourself!

  • Run swirl() and finish the following lessons of the R Programming course:
    • Lesson 6: Subsetting vectors
    • Lesson 12: Looking at data

Statistics

  • Goal of statistics is to gain understanding from data
    • Descriptive statistics (this lecture): describe data without further conclusions
    • Inferential statistics: describe data (sample) and its relation to larger group (population)

Numerical variables: central tendency and spread

mean(dat$Diff)  # mean
# [1] 0.016263
median(dat$Diff)  # median
# [1] 0.01093
min(dat$Diff)  # minimum value
# [1] -0.042507
max(dat$Diff)  # maximum value
# [1] 0.10346

Numerical variables: measures of spread

sd(dat$Diff)  # or: sqrt((1/(length(dat$Diff)-1)) * sum((dat$Diff - mean(dat$Diff))^2))
# [1] 0.038213
var(dat$Diff)  # or: sd(dat$Diff)^2
# [1] 0.0014603
quantile(dat$Diff)  # quantiles
#         0%        25%        50%        75%       100% 
# -0.0425067 -0.0038419  0.0109299  0.0248903  0.1034607
summary(dat$Diff)  # summary
#     Min.  1st Qu.   Median     Mean  3rd Qu.     Max. 
# -0.04251 -0.00384  0.01093  0.01626  0.02489  0.10346

Categorical variables: frequency tables

table(dat$Gender)
# 
#  F  M 
#  9 10
with(dat, table(Gender))  # alternative
# Gender
#  F  M 
#  9 10
table(dat$DiffClass)
# 
# TH- TH0 TH1 
#   6   7   6

Question 7

Exploring relationships between pairs of variables

# correlation: relation between two numerical variables
cor(dat$Frontness.T, dat$Frontness.TH)
# [1] 0.71054
# crosstable: relation between two categorical variables
table(dat$Gender, dat$DiffClass)  # or: with(dat, table(Gender,DiffClass))
#    
#     TH- TH0 TH1
#   F   1   3   5
#   M   5   4   1
# means per category: relation between numerical and categorical variable
c(mean(dat[dat$Gender == "M", ]$Diff), mean(dat[dat$Gender == "F", ]$Diff))
# [1] -0.0034299  0.0381446

Question 8

Data exploration with visualization

  • Many basic visualization options are available in R
    • boxplot() for a boxplot
    • hist() for a histogram
    • qqnorm() and qqline() for a quantile-quantile plot
    • plot() for many types of plots (scatter, line, etc.)
    • barplot() for a barplot (plotting frequencies)

Exploring numerical variables: box plot

par(mfrow = c(1, 2))  # set graphics option: 2 graphs side-by-side
boxplot(dat$Diff, main = "Difference")  # boxplot of difference values
boxplot(dat[, c("Frontness.T", "Frontness.TH")])  # frontness per group

plot of chunk unnamed-chunk-17

Exploring numerical variables: histogram

hist(dat$Diff, main = "Difference histogram")

plot of chunk unnamed-chunk-18

Exploring numerical variables: Q-Q plot

qqnorm(dat$Diff)  # plot actual values vs. theoretical quantiles
qqline(dat$Diff)  # plot reference line of normal distribution

plot of chunk unnamed-chunk-19

Exploring numerical relations: scatter plot

plot(dat$Frontness.T, dat$Frontness.TH, col = "blue")

plot of chunk unnamed-chunk-20

Visualizing categorical variables (frequencies): bar plot

counts <- table(dat$Gender)  # frequency table for gender
barplot(counts, ylim = c(0, 15))

plot of chunk unnamed-chunk-21

Exploring categorical relations: segmented bar plot

counts <- table(dat$Gender, dat$DiffClass)
barplot(counts, col = c("pink", "lightblue"), legend = rownames(counts), ylim = c(0, 
    10))

plot of chunk unnamed-chunk-22

Question 9

Try it yourself!

  • Run swirl() and finish the following lesson of the R Programming course:
    • Lesson 15: Base graphics

Recap

  • In this lecture, we've covered the basics of R
  • Now you should be able (with help of this presentation) to use R for:
    • Data manipulation, exploration and visualization
  • Associated lab session and additional swirl resources:

Evaluation

Questions?

Thank you for your attention!

http://www.martijnwieling.nl
m.b.wieling@rug.nl