Percentiles !?!?!?!

Hi,



I have a field called 'response time'. In my report, I want to select only the data which is within the fastest 10percent of response times. So, I entered a record selection formula similar to:

{log.responseTime} < PthPercentile(10,{log.responseTime})

However, when I try to save this, it says something like 'Formula cannot be used because it must be evaluated later' (I'm not sure if that is the exact wording).

Am I doing something wrong? Is there another way to do what I want to do.

Thanks