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Multiple selection and normalization
2 messages · Robert Lynch, Rui Barradas
Hello, A data example would be great. Try the following. set.seed(71926) # make up some data dat <- data.frame(SIDN = 1:100, TERM = factor(sample(4, 100, TRUE)), GRADE = runif(100), INST = factor(sample(5, 100, TRUE))) head(dat) with(dat, ave(GRADE, list(TERM, INST), FUN = scale)) Hope this helps, Rui Barradas Em 03-06-2013 21:23, Robert Lynch escreveu:
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 [[alternative HTML version deleted]]
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