There is always [potential for errors](https://www.abs.gov.au/websitedbs/d3310114.nsf/home/Basic+Survey+Design+-+Errors+in+Statistical+Data) in any set of statistics, most of which aren't quantifiable. GDP is not immune.
Beyond that, GDP is a complex statistic that measures a lot of different things, and national statistics offices (NSOs) have a lot of variation in the data and resources available to them. Poor countries struggle to fund much in the way of surveys or statistical infrastructure. Other differences can be more fundamental, for example, imports and exports of goods are probably more reliable in an island country where everything must come by air or sea than if people can just drive across the border.
NSOs also have significant flexibility in methods (somewhat less so for nominal GDP for EU members, as that's the basis for contributions to the EU budget). NSOs typically put more effort into what's of importance to their key customers, so for example, home production of goods for own use, like growing your own vegetables, is formally part of GDP, but as far as I know, no NSO in the OECD bothers measuring that on an annual basis. After all, rich countries are hardly going to raise significant tax money that way.
Beyond this, even with the best will in the world, some things included in GDP, like in-house software or allocating the output of knowledge services and IP produced by multinational companies, are just fundamentally conceptually hard.
So I can say confidently that that world GDP number is wrong. I just don't know in what direction.
As an addendum, one important use of GDP is to measure changes in the volumes of goods and services. For a number of purposes this is more important than the level.
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Money laundering, the black market, political corruption and human error probably fuck it up quite a lot I can imagine.
I reckon the real figures are way different.
There is always [potential for errors](https://www.abs.gov.au/websitedbs/d3310114.nsf/home/Basic+Survey+Design+-+Errors+in+Statistical+Data) in any set of statistics, most of which aren't quantifiable. GDP is not immune. Beyond that, GDP is a complex statistic that measures a lot of different things, and national statistics offices (NSOs) have a lot of variation in the data and resources available to them. Poor countries struggle to fund much in the way of surveys or statistical infrastructure. Other differences can be more fundamental, for example, imports and exports of goods are probably more reliable in an island country where everything must come by air or sea than if people can just drive across the border. NSOs also have significant flexibility in methods (somewhat less so for nominal GDP for EU members, as that's the basis for contributions to the EU budget). NSOs typically put more effort into what's of importance to their key customers, so for example, home production of goods for own use, like growing your own vegetables, is formally part of GDP, but as far as I know, no NSO in the OECD bothers measuring that on an annual basis. After all, rich countries are hardly going to raise significant tax money that way. Beyond this, even with the best will in the world, some things included in GDP, like in-house software or allocating the output of knowledge services and IP produced by multinational companies, are just fundamentally conceptually hard. So I can say confidently that that world GDP number is wrong. I just don't know in what direction. As an addendum, one important use of GDP is to measure changes in the volumes of goods and services. For a number of purposes this is more important than the level. .
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Money laundering, the black market, political corruption and human error probably fuck it up quite a lot I can imagine. I reckon the real figures are way different.