Week 2: Data & Variables

POP88162 Introduction to Quantitative Research Methods

Useful mathematical functions in R

sum(<numbers>) # Sum
mean(<numbers>) # Mean
median(<numbers>) # Median
sd(<numbers>) # Standard Deviation
log(<number>) # Logarithm
exp(<number>) # Exponent
sqrt(<number>) # Square root

Working with data

Download the dataset on inter-state wars from The Correlates of War Project. This dataset is also available on Blackboard under section “Data”. The codebook for this dataset is available here.

Read it into R using read.csv function or read_csv from readr package.

wars <- read.csv("Your file path")

Check the top and bottom of the data.

head(wars)
tail(wars, 20)

How many observations are there? And how many variables?

str(wars)
dim(wars)
nrow(wars)
ncol(wars)

What constitutes an individual observation?

Working with variables

Check how many unique wars are listed in the dataset.

length(unique(<dataset>$<variable_name>))

First, select the variable which records start years of each war (StartYear1).

Now select the variable which records end years of each war (EndYear2).

Create a new variable, which shows the duration of each war in years by subtracting start years from end years. You can call it something like war_duration.

Keep only unique wars in the dataset.

unique_wars <- wars[unique(wars$WarNum),]

Calculate the mean and median of this new variable. What is the mean and what is the median duration of inter-state wars in this dataset in years.