Automatically sorting `by' ---------------------------
^bys^ varlist [^(^varlist2^)^] ^:^ command
Description ------------
By ^bys^ command is very similar to ^by varlist: command^ but automatically sor > ts on the variables in varlist.
If a second varlist, varlist2, is specified between parentheses, ^bys^ will sort the data on varlist2 within varlist. This may be required for some more complicated commands, in particular to generate variables within subgroups (see example 2 below).
If the data are already sorted on varlist and further variables, the sort order is not changed. The command may not contain quotes (").
Example --------
Ex 1. . ^bys age: summ inc^
is equivalent to
. ^sort age^ . ^by age: summ inc^ Ex 2. We want to assign to people their rank-number with respect to age in their own sex group.
. ^bys sex (age): gen rank = _n^
would be equivalent to
. ^sort sex age^ . ^by sex: gen rank = _n^
In this last 2-line solution it may not be directly apparent that the by: construct only works correctly if the data were sorted appropriately within the by-groups. Using ^bys^ makes this more transparent.
Note that the equivalence in both examples is not perfect: ^bys^ will not sort the data if they have already been sorted on the required variables. Thus, if the data were sorted on more variables, the original sorting order and Stata's information about ordering will be left unchanged.
Author -------
Jeroen Weesie Dept of Sociology/ICS Utrecht University J.Weesie @@ fss.uu.nl
Also see ---------
Manual: [2] by On-line: ^help^ for @by@