Prior to Oracle 10g, adjusting optimizer parameters was the only way to compensate for sample size issues with dbms_stats. As of Oracle 10g, the use of dbms_stats.gather_system_stats and improved sampling within dbms_stats have made adjustments to these parameters less important. With other things the same, always adjust CBO statistics before adjusting optimizer parameters.
The CBO gathers information from many sources, and he has the goal of using DBA-provided metadata to make the most desireable execution plan decision:
Let us examine the following areas of CBO statistics and see how to gather quality statistics for the CBO and how to create an appropriate CBO environment for your database.

The choices of executions plans made by the CBO are only as good as the statistics available to it. The old-fashioned analyze table and dbms_utility methods for generating CBO statistics are obsolete and somewhat dangerous to SQL performance. As we may know, the CBO uses object statistics to choose the best execution plan for all SQL statements.
The dbms_stats utility does a far better job in estimating statistics, especially for large partitioned tables, and the better statistics result in faster SQL execution plans. Here is a sample execution of dbms_stats with the OPTIONS clause:

 
exec dbms_stats.gather_schema_stats( - 
 ownname          => 'SCOTT', - 
 options          => 'GATHER AUTO', - 
 estimate_percent => dbms_stats.auto_sample_size, - 
 method_opt       => 'for all columns size repeat', - 
 degree           => 34 - 
 )

BEGIN
dbms_stats.gather_schema_stats(
ownname=>'TPCC',
METHOD_OPT=>'FOR ALL INDEXED COLUMNS SIZE SKEWONLY',
CASCADE=>TRUE,
ESTIMATE_PERCENT=>100);
END;
/

DBMS_STATS.CREATE_STAT_TABLE
('scott','savestats');