Scientists compare rock layers before and after a mass extinction to estimate the number of affected species. They make estimates from several rock layer samples. Then they compare the loss of species to the normal rate of extinction that happens as life changes over time. This allows scientists to estimate the percentage of species on Earth affected by a mass extinction. The number of extinctions over time can be graphed. These graphs show a relatively constant extinction rate, broken up by large changes in the number of families of organisms lost during mass extinction events.