Hi:
It seems to me that another way to handle this problem easily would be to
add a variable indicating the 2009 school before converting it to a data
table. The function is simple enough: find the max SCHOOL_NUMBER per
STUDENT_ID and use ave() to map it to all rows with the same STUDENT_ID.
library(data.table)
txt$SCHOOL2009 <- with(txt, ave(SCHOOL_NUMBER, STUDENT_ID, FUN = max))
dtxt <- data.table(txt)
dtxt[YEAR == 2008, mean(SCORE), by = 'SCHOOL2009']
SCHOOL2009 V1
[1,] 200 48.2
[2,] 400 53.0
As this is my first posting on this list, how do I follow-up to an existing
thread?
Dennis
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