Because id is a factor in your data frame, and the levels (including "C") is kept when subsetted. Here is one way to get ride of "C".
> ex1$id <- factor(ex1$id) > tapply(ex1$x, ex1$id, mean) A B 22.5 32.5 On Sat, May 10, 2008 at 11:23 AM, Dipankar Basu <[EMAIL PROTECTED]> wrote: > Hi! > > I am using R version 2.7.0 and am working on a panel dataset read into R as > a dataframe; I call it "ex". The variables in "ex" are: id year x > > id: a character string which identifies the unit > year: identifies the time period > x: the variable of interest (which might contain NAs). > > Here is an example: >> id <- rep(c("A","B","C"),2) >> year <- c(rep(1970,3),rep(1980,3)) >> x <- c(20,30,40,25,35,45) >> ex <- data.frame(id=id,year=year,x=x) >> ex > id year x > 1 A 1970 20 > 2 B 1970 30 > 3 C 1970 40 > 4 A 1980 25 > 5 B 1980 35 > 6 C 1980 45 > > I want to draw a subset of "ex" by selecting only the A and B units: > >> ex1 <- subset(ex[which(ex$id=="A"|ex$id=="B"),]) > > Now I want to do some computations on x for each selected unit only: > >> tapply(ex1$x, ex1$id, mean) > A B C > 22.5 32.5 NA > > But this gives me an NA value for the unit C, which I thought I had already > left out. How do I ensure that the computation (in the last step) is limited > to only the units I have selected in the first step? > > Dipankar > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- HUANG Ronggui, Wincent Bachelor of Social Work, Fudan University, China Master of sociology, Fudan University, China Ph.D. Candidate, CityU of HK, http://www.cityu.edu.hk/sa/psa_web2006/students/rdegree/huangronggui.html ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.