EXP-00091: Exporting questionable statistics

When  I export  one of  database schema by one of Oracle client machine, I have received following warning message    “EXP-00091: Exporting questionable statistics” and  schema successfully exported. But My question is why shown the above warning message . I have searching in google  and found possible cause of error and solutions my exporting  problem.

Cause :

 => There are row errors while exporting
 =>The client character set or NCHAR character set does not match the server character set or NCHAR character set
=>A QUERY clause is specified
=>Only certain partitions or sub partitions are exported 

Solution :

When OS character set different that ORACLE character set then BEFORE export you should set NLS_LANG=.CHARACTERSET according Oracle Database Character set.
Example : Oracle 10gr2 on Linux version 4
OS NLS_LANG = US7ASCII
Oracle NLS_LANG = WE8IS0XXXXX

Just Before export set NLS_LANG like
$export NLS_LANG=.WE8ISOXXXXX
$export your_data

Example on windows

First check "NLS_CHARACTERSET" of database through below query

SQL> select VALUE
2 from nls_database_parameters
3 where PARAMETER = 'NLS_CHARACTERSET';

VALUE
----------------------------------------
WE8MSWIN1252


On command prompt

c:\> set NLS_LANG=.WE8MSWIN1252