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]]
>
> ______________________________________________
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> 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
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