Counting unique items in data frame

Question

I want a simple count of the number of subjects in each condition of a study. The data look something like this:

subjectid  cond   obser variable
1234                


        

Answer 1

Using your snippet of data that I loaded into object dat:

> dat
  subjectid cond obser variable
1      1234    1     1       12
2      1234    1     2       14
3      2143    2     1       19
4      3456    1     1       12
5      3456    1     2       14
6      3456    1     3       13

Then one way to do this is to use aggregate to count the unique subjectid (assuming that is what you meant by "Ss"???

> aggregate(subjectid ~ cond, data = dat, FUN = function(x) length(unique(x)))
  cond subjectid
1    1         2
2    2         1