Accuracy
The fact that we do not currently know the total number of species, in the past nor the present, makes it very difficult to accurately calculate the non-anthropogenically influenced extinction rates. As a rate, it is essential to know not just the number of extinctions, but also the number of non-extinctions. This fact, coupled with the fact that the rates do not remain constant, significantly reduces accuracy in estimates of the normal rate of extinctions.
Read more about this topic: Background Extinction Rate
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