The ratio of the capital used in a process, a firm, or an industry to output over some period, usually a year. This ratio for any process depends on the relative cost of different inputs. Where technology makes alternative techniques feasible, firms usually choose the cheapest, so capital–output ratios tend to be high when capital is cheap relative to other inputs. For a firm or an industry, the capital–output ratio will depend on the mix of different outputs produced and different processes used. The capital–output ratio can be measured as an average ratio, comparing total capital stock with total output, or as a marginal ratio, comparing increases in capital used with increases in output. Both average and marginal capital–output ratios are taken to refer to normal levels of working: when output is abnormally low during a recession, the average capital–output ratio is unusually high, but the marginal capital–output ratio is unusually low.