The cost of living is the cost of maintaining a certain standard of living for an individual or a household. Changes in the cost of living over time can be measured in a cost-of-living index. Cost of living calculations are also used to compare the cost of maintaining a certain standard of living in different geographic areas. Differences in the cost of living between locations can be measured in terms of purchasing power parity rates. A sharp rise in the cost of living can trigger a cost of living crisis, where purchasing power is lost and, for some people, their previous lifestyle is no longer affordable.