Multiple selection and normalization

Hi--

I am trying to normalize course grades for each instance of a course, e.g.
Stats 1 Fall2009 J. Smith.

I have a frame for all instances of a course, e.g. stats 1 in the last 5
years, that looks like

SIDN  TERM  GRADE  INST

where SIDN is a Student ID Number, TERM is a factor that gives the quarter
and year a course was offered, GRADE is a 0-4.3 grade and INST is the
instructor, again as a factor.

Course offerings are determined by the TERM and INST.  That is one inst.
assigned grades to all the students they were responsible for that term.
  Multiple instructors may have taught the same term.

For every course offering I would like to normalize the GRADE:  Z<- (GRADE
- mean)/SD  where the mean and SD are over a single course offering.

Thanks!
RBL

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