dbc.ampusage - response (3) by trustngs

Hi Dieter,
SELECT SUM(DiskIO) / SUM(CpuTime) AS IO_Per_CPU_Second FROM DBC.AMPUsage;
An average query for the system has an IO_Per_CPU_Second of 1000.
Now, how to identify which queries are canditates for performce tuning? Say the queries operating at approximately 1000 (or) queries operating at 100 or lower (or) queries operating at greater than or equal to 2500. 
Please advise.