[R] Percentages for categorical data by group

[Marc Schwartz]

on 05/23/2008 09:51 AM Economics Guy wrote:
> I can think of several ways to blunt force hard code what I want but I
> imagine there is a command or two that can be easily combined to do this:
> 
> I have a data frame with about 23000 observations. There first variable is
> the group to which the observation belongs (about 500 different groups). The
> second variable is a response for each observation that is a 1,2,3,4 or 5. I
> want to be able to calculate the percentage of each group that choose each
> response. For example I want to know what percentage of group 1 (which may
> have a value of 34456) choose response 1 and so on.
> 
> Here is some code I wrote that generates a data frame like the one I have.
> 
> pop <- matrix(1:100000)
> groupIDs <- sample(pop,500)
> groupVar <- sample(groupIDs,23000,replace=TRUE)
> responseVar <- sample(1:5,23000,replace=TRUE)
> 
> example.data <- data.frame(groupVar,responseVar)
> 
> Is there a fast way to calculate these percentages beyond writing loops to
> manually count the responses for each of the groups?
> 
> Thanks,
> 
> EG

Using:

   table(example.data)

will give you a cross tabulation of the counts of your ResponseVar by 
each groupVar.

   prop.table(table(example.data), 1)

will give you a row-wise proportion (0 - 1) of the counts of ResponseVar 
for each groupVar. If you want percentages (0 - 100):

    prop.table(table(example.data), 1) * 100


See ?table and ?prop.table for more information.

HTH,

Marc Schwartz