In order to nest aggregation functions, you need to use "Advanced Aggregation" with function "Aggr". You also need to determine, at what level of detail do you want the "internal" aggregation to happen - i.e. you are comparing RTT with the average of PTT - at what level?
when everything is defined correctly, it should work. Function aggr() takes in account your selections.
You need to answer this question: "Avg(PTT)" - for what Entity? Average by Port? or Country? If you are after the overall average (for anything selected/associated), than the approach with the variable (suggested above) would work even better.
Sergio, We could've seen it - if you could post an example. I"m sure that aggr will work if you correctly define the dimensions which you need to use in aggregation. Sometimes it's not straightforward.