Of course no such measurement can be exact. There is systematic error. I don't think that means the number is without value though. I would have to assume the numbers used for the calculation are the result of a survey. Just to start with, there is error in a sample size relating to the whole.
If we want to go all the way, even a number such as GDP is not precise. The assumption is that the invisible money of last quarter will be the invisible money of this quarter.
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