第一步：　查看DB中的NLS_CHARACTERSET的值（提供两种方法）：

查询NLS_CHARACTERSET的值：
select * from nls_database_parameters t where t.parameter='NLS_CHARACTERSET'
or
select * from v$nls_parameters  where parameter='NLS_CHARACTERSET';
SQL> select * from v$nls_parameters where parameter='NLS_CHARACTERSET';
PARAMETER　　　　      VALUE
-----------------------　　----------------------------------------------
NLS_CHARACTERSET    ZHT16BIG5
第二步：　根据第一步查出的NLS_CHARACTERSET（ZHT16BIG5）來設定exp的環境變量：

WINNT> set NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5
进行设定后，在环境变量中查看NLS_LANG的值是否一致，如果不一致可在环境变量中手中修改。
LINUX> export NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5

oerr exp 91
00091, 00000, "Exporting questionable statistics."
// *Cause:  Export was able export statistics, but the statistics may not be
//          usuable. The statistics are questionable because one or more of
//          the following happened during export: a row error occurred, client
//          character set or NCHARSET does not match with the server, a query
//          clause was specified on export, only certain partitions or
//          subpartitions were exported, or a fatal error occurred while
//          processing a table.
// *Action: To export non-questionable statistics, change the client character
//          set or NCHARSET to match the server, export with no query clause,
//          export complete tables. If desired, import parameters can be
//          supplied so that only non-questionable statistics will be imported,
//          and all questionable statistics will be recalculated.