It's not so much a matter of being overconfident as it is not listing the disclaimers at every opportunity. The Laspeyres Price Index (the usual type of price index) has well understood limitations (specifically that it overestimates consumer price growth as it doesn't deal with technological improvement and substitution effects very well), but since we don't have anything better, we use it anyway.
"Real" is a term of art in economics. It's used to reflect inflation-adjusted figures because all nominal GDP tells you is how much money is floating around, which isn't all that useful. real GDP may be less certain, but it's more useful.
Bear in mind that everything economists use is an estimate of a sort, even nominal GDP. Believe it or not, they don't actually ask every business in the country how much they produced and / or received in income (which is why the income and expenditure methods of calculating GDP give slightly different numbers although they should give exactly the same result in theory). The reason this may not be readily apparent is that most non-technical audiences start to black out the moment you talk about calculating a price index (hell, it makes me drowsy) and technical audiences already understand the limitations.
James_K:
"Real" is a term of art in economics. It's used to reflect inflation-adjusted figures because all nominal GDP tells you is how much money is floating around, which isn't all that useful. real GDP may be less certain, but it's more useful.
You're talking about the "real" figures being "less certain," as if there were some objective fact of the matter that these numbers are trying to approximate. But in reality, there is no such thing, since there exists no objective property of the real world that would make one way ...
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