I am working with a longitudinal data set in the long format. This data
set has three observations per grade level per year. Here are the first
10 rows of the data frame:
>tenn.dat[1:10,]
year schid type grade gain se new cohort
6 2001 100005 5 4 33.1 3.5 4 3
7 2002 100005 5 4 33.9 3.9 4 2
8 2003 100005 5 4 32.3 4.2 4 1
10 2001 100005 5 5 22.9 4.0 5 4
11 2002 100005 5 5 25.0 3.4 5 3
12 2003 100005 5 5 7.8 3.8 5 2
18 2001 100010 1 4 34.4 5.9 4 3
19 2002 100010 1 4 27.8 5.6 4 2
20 2003 100010 1 4 34.6 6.8 4 1
22 2001 100010 1 5 21.1 4.8 5 4
I need to create a new column in this data frame with the mean gain for
each grade by year and the sd for each grade by year.
So, I used tapply as follows:
tapply(tenn.dat[,5],tenn.dat[,c(1,4)],mean) and
tapply(tenn.dat[,5],tenn.dat[,c(1,4)],sd) which produces exactly the
data I would like to attach in column 1 and 2 respectively.
I am having a problem connecting this back with the corresponding rows
in the data frame.
If I used only one factor instead of two, I was successful connecting
this with the data frame using:
m.gain<-tapply(tenn.dat[,5],tenn.dat[,4],mean)
tenn.dat$m.gain<-m.gain[as.character(tenn.dat$grade)]
can anyone offer suggestions on a next step?
Thanks,
Harold
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