[1] Background extinction rates have not remained constant, although changes are measured over geological time, covering millions of years.
[2][3][4] Background extinction rates are typically measured in order to give a specific classification to a species and this is obtained over a certain period of time.
[5] The first is simply the number of species that normally go extinct over a given period of time.
For example, at the background rate one species of bird will go extinct every estimated 400 years.
[6] Another way the extinction rate can be given is in million species years (MSY).