On Tue, 5 Jan 2010, Geoffrey Smith wrote:
Hello, does anyone know how to take the mean for a subset of observations?
For example, suppose my data looks like this:
OBS NAME SCORE
1 Tom 92
2 Tom 88
3 Tom 56
4 James 85
5 James 75
6 James 32
7 Dawn 56
8 Dawn 91
9 Clara 95
10 Clara 84
Is there a way to get the mean of the SCORE variable by NAME but only when
the number of observations is equal to 3? In other words, is there a way to
get the mean of the SCORE variable for Tom and James, but not for Dawn and
Clara? Thank you.
You can use tapply() together with a custom function that returns NA if
the condition is not satisfied, e.g.
## read data
dat <- read.table(textConnection("
OBS NAME SCORE
1 Tom 92
2 Tom 88
3 Tom 56
4 James 85
5 James 75
6 James 32
7 Dawn 56
8 Dawn 91
9 Clara 95
10 Clara 84
"), header = TRUE)
## use tapply() with custom function
with(dat,
tapply(SCORE, NAME, function(x) if(length(x) == 3) mean(x) else NA)
)
Alternatively you could look at
mymean <- with(dat, tapply(SCORE, NAME, mean))
mylength <- with(dat, tapply(SCORE, NAME, length))
mymean[mylength == 3]
etc.
hth,
Z
--
Geoffrey Smith
Visiting Assistant Professor
Department of Finance
W. P. Carey School of Business
Arizona State University
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