# Preliminaries

xtabs() is the numerical version of barchartGC(). You use it when you want to study

• the distribution of one factor variable;
• the relationship between two factor variables.

The function xtabs() comes with the stats package, which is always loaded in R. However, some of the data and other functions that we will use come from the tigerstats package, so make sure that it is loaded:

require(tigerstats)

# One Factor Variable

To see a table of the tallies for the factor variable seat (from the mat111survey data frame in the tigerstats package):

xtabs(~seat,data=m111survey)
## seat
##  1_front 2_middle   3_back
##       27       32       12

In order to get the actual distribution of seat, you want percents rather than counts, so apply the function rowPerc() from tigerstats:

rowPerc(xtabs(~seat,data=m111survey))
##
## seat 1_front 2_middle 3_back Total
##        38.03    45.07   16.9   100

If you have a table of the counts for a variable, then you can enter it directly. For example, suppose you have already made:

Seat <- xtabs(~seat,data=m111survey)
Seat
## seat
##  1_front 2_middle   3_back
##       27       32       12

Then you can just enter the table into rowPerc():

rowPerc(Seat)
##
## seat 1_front 2_middle 3_back Total
##        38.03    45.07   16.9   100

# Two Factor Variables

You can also use xtabs() to study the relationship between two factor variables. For example, if you want to see whether males and females differ in their seating preferences, then you might try formula-data input as follows:

xtabs(~sex+seat,data=m111survey)
##         seat
## sex      1_front 2_middle 3_back
##   female      19       16      5
##   male         8       16      7

Of course, row percents are the way to actually study the relationship:

rowPerc(xtabs(~sex+seat,data=m111survey))
##         seat
## sex      1_front 2_middle 3_back  Total
##   female   47.50    40.00  12.50 100.00
##   male     25.81    51.61  22.58 100.00

Note the type of formula used to study the relationship between two factor variables:

$\sim ExplanatoryFactor + ResponseFactor$

Note also that people usually want to do several things with their two-way table, so often they will make it and store it in a well-named object, and then print out the object:

SexSeat <- xtabs(~sex+seat,data=m111survey)
SexSeat
##         seat
## sex      1_front 2_middle 3_back
##   female      19       16      5
##   male         8       16      7

Then to get row percents they just put in the named object:

rowPerc(SexSeat)
##         seat
## sex      1_front 2_middle 3_back  Total
##   female   47.50    40.00  12.50 100.00
##   male     25.81    51.61  22.58 100.00

# Column Percents, Too!

We donâ€™t use column percents very often, but you can get them:

colPerc(SexSeat)
##         seat
## sex      1_front 2_middle 3_back
##   female   70.37       50  41.67
##   male     29.63       50  58.33
##   Total   100.00      100 100.00