Depending on what you are actually trying to find out, you may also need to look into how that wealth is distributed. GDP doesn't mean much for most if 99.9% of that wealth is just owned by .01% of the population. Cost of living is also significant. Plus there are other externalities. How much use is it being rich if you can't step outside because the air is so polluted? Does the population have to work on average 80 hours a week to achieve that GDP? How much corruption is there? etc..
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Depending on what you are actually trying to find out, you may also need to look into how that wealth is distributed. GDP doesn't mean much for most if 99.9% of that wealth is just owned by .01% of the population. Cost of living is also significant. Plus there are other externalities. How much use is it being rich if you can't step outside because the air is so polluted? Does the population have to work on average 80 hours a week to achieve that GDP? How much corruption is there? etc..
But of course, if you are trying to find out if they can afford to clean up their rivers, GDP is probably a fairly good measure..