唉。
help吧。

rank(exp) [, [field|track|unique]]   (allows by varlist:)  
creates ranks (within varlist) of exp; by default, equal observations are assigned the average rank.  The field option calculates the field rank of exp: the highest value is ranked 1, and there is no correction for ties.  That is, the field rank is 1 + the number of values that are higher.  The track option calculates the track rank of exp:  the lowest value is ranked 1, and there is no correction for ties.  That is, the track rank is 1 + the number of values that are lower.  The unique option calculates the unique rank of exp: values are ranked 1,...,#, and values and ties are broken arbitrarily.  Two values that are tied for second are ranked 2 and 3.

谢谢可爱的版主。