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
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