nobody really answered my question, so I'll see if I can try it again. Frequently in my games I'm the best in GDP and GNP, but in my most recent game I had a one star economy. How is that possible? If I have the best economy and mine is only one star, I can only imagine what is happening to the other civs. By the way, I was reading the stability guide on the wiki on the economy, and how can I know if I have too high a population? should I start cutting off pop growth before it reaches the limit?
I am no expert in this, but AIUI "economy" in rfc stability terms includes all the things you get from tiles, ie. food, hammers and comerce, and to have a good economy this needs to keep expanding. So if you have a burst of growth them stagnation you will have a low economy score compared to if you you have steady low level growth, even if you have an identical empire in the 2 saenarios.
You can find more details at http://wikirhye.wikidot.com/stability#toc35
[EDID] Sorry, I just read your post again (did you edit or am i blind?). In my experience you want to keep your population growing steadily, and you have a good economy rating. I only get a bad one when I do not have groth, for example if I have a steady happiness cap for a long time, or if I am using the whip heavily.