呵呵，建议你用word
help limits 可查看Stata受限清单

要用stata分析那些系列的分布特征

变量的分布也取值的长度也没什么关系啊

要算出不同的字母在序列中出现的频率，如何处理

You can dowload the user written program -egenmore-, which provides several functions that fit your need.
e.g.

incss(strvarlist) , substr(substring) [ insensitive ] indicates occurrences of
        substring within any of the variables in a list of string variables by 1 and
        other observations by 0. insensitive makes comparison case-insensitive.
        (Stata 6 required; an alternative is now just to use foreach.)

noccur(strvar) , string(substr) creates a variable containing the number of
        occurrences of the string substr in string variable strvar.  Note that
        occurrences must be disjoint (non-overlapping): thus there are two
        occurrences of "aa" within "aaaaa". (Stata 7 required.)

    nss(strvar) , find(substr) [ insensitive ] returns the number of occurrences of
        substr within the string variable strvar.  insensitive makes counting
        case-insensitive. (Stata 6 required.)