Using R for Introductory Statistics, 3.1

May 21, 2010
By

(This article was first published on Digithead's Lab Notebook, and kindly contributed to R-bloggers)

Pairs of categorical data

The grades data.frame holds two columns of letter grades, giving pairs of categorical data, like so:

    prev grade
1    B+    B+ 
2    A-    A- 
3    B+    A- 
...
122  B     B

This type of data can be summarized by the table function, which counts the occurrence of each possible pair of letter grades. But first, I was never a fan of plus-minus grading, so lets do away with that.

> grades2 <- data.frame( prev=factor(gsub("[+]|-| ", "", as.character(grades$prev)), levels=c('A','B','C','D','F')), grade=factor(gsub("[+]|-| ", "", as.character(grades$grade)), levels=c('A','B','C','D','F')) )
> table(grades2)
    grade
prev  A  B  C  D  F
   A 22  6  3  2  0
   B  4 15  5  1  3
   C  3  2  9  9  7
   D  0  1  4  3  1
   F  1  2  4  4 11

You might want to compute row (1) or column (2) sums, using margin.table:

> margin.table(table(grades2), 1)
prev
 A  B  C  D  F 
33 28 30  9 22 

Of the students who got an A on the first test, what proportion also got an A on the second test? Those types of questions are answered by prop.table().

> options(digits=1)
> prop.table(table(grades2), 1)
    grade
prev    A    B    C    D    F
   A 0.67 0.18 0.09 0.06 0.00
   B 0.14 0.54 0.18 0.04 0.11
   C 0.10 0.07 0.30 0.30 0.23
   D 0.00 0.11 0.44 0.33 0.11
   F 0.05 0.09 0.18 0.18 0.50
> options(digits=4)

Finally, this type of data can be displayed as a stacked barplot.

m <- t(as.matrix(florida[,2:3]))
m.prop <- prop.table(m, margin=2)
colnames(m.prop) <- florida$County

# fool around with margins and set style of axis labels
# mar=c(bottom, left, top, right)
# las=2 => always perpendicular to the axis
old = par(mar=c(6,4,6,2)+0.1, las=2)

# cex.names => "character expansion" of bar labels
# args.legend => position the legend out of the plot area
barplot(m.prop[,order(m.prop[2,])], legend.text=T, cex.names=0.40, args.legend=list(x=82,y=1.2), main="2000 Election results in Florida", sub='county')

# reset old parameters
par(old)

To leave a comment for the author, please follow the link and comment on their blog: Digithead's Lab Notebook.

R-bloggers.com offers daily e-mail updates about R news and tutorials on topics such as: Data science, Big Data, R jobs, visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...



If you got this far, why not subscribe for updates from the site? Choose your flavor: e-mail, twitter, RSS, or facebook...

Tags: , ,

Comments are closed.

Sponsors

Mango solutions



RStudio homepage



Zero Inflated Models and Generalized Linear Mixed Models with R

Dommino data lab

Quantide: statistical consulting and training



http://www.eoda.de







ODSC

ODSC

CRC R books series





Six Sigma Online Training





Contact us if you wish to help support R-bloggers, and place your banner here.

Never miss an update!
Subscribe to R-bloggers to receive
e-mails with the latest R posts.
(You will not see this message again.)

Click here to close (This popup will not appear again)