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2011 Dec 04
3
Group several variables and apply a function to the group
..., x=x) For each company, within a particular month, I would like to compute the standard deviation of x: for example, for abc, I would like to compute the sd of x for month1 (when mi=1) and for month2 (when mi=2). In other languages (Stata for instance), I would create a grouping variable (group comnn and mi) and then, apply the sd function for each group. However, I don't find an elegant way to do the same in R: I was thinking about the following: I could subset my data frame by mi and create one file per month, and then make a loop and in each file, use a "by" operator for each...