On Dec 20, 2009, at 4:34 AM, jie feng wrote:
Dear R experts:
I have following data structure: student id, exam time and score. I
want to
create three new columes: 1 st before, 2nd before and 3rd before.
For example, for student 1's 4th exam score is assumed to be
influenced by
his previous three cloest scores, which are 6, 9, and 10 in that
order.
For student 2's 5th exam score is assumed to be inflenced by her
previous
cloest three scores, which are 10, 10 and 8 in that order.
Note that for student 1, his first time score "10" can not be
influenced by
other scores, thus has three empties in the three new columes.
for student 1, his second score "9" is only influenced by his first
score
thus has two empties in the columes of 2nd before and 3rd before.
Now I only have 3 columes: id, time and score, and I want to create
three
new columes: 1st before, 2nd before and 3rd before. How can I do that?
Thanks a lot.
The combination of your choice of text editors and your use of HTML
mail format (contrary to the Posting Guide ... please read it more
carefully) has completely mangled your posting of the data. As a
result it is not possible for ordinary humans to figure out what your
starting point really is. It's possible that the answer lies in the
help page for lag:
?lag
Future postings on this matter ought to include the data in parse-able
R format (also described in the Posting Guide.)
Best
Jay
id time score 1st before 2nd
before 3rd before
1 1 10 1 2 9 10 1 3 6 9 10 1 4 10 6 9 10 1 5 5 10 6 9 2 1
7 2 2 8
7 2 3 10 8 7 2 4 10 10 8 7 2 5 5 10 10 8 2 6 9 5 10 10
http://www.R-project.org/posting-guide.html
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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