Energy quality is a measure of the ease at which a form of energy can be converted to useful work or to another form of energy. A high quality form of energy is easily converted to work or to a lower quality form of energy, whereas converting low quality forms of energy to work or a higher quality form may be inefficient, difficult or impossible. The concept of energy quality is also used in ecology, where it is used to track the flow of energy between different trophic levels in a food chain and in thermoeconomics, where it is used as a measure of economic output per unit of energy. Methods of evaluating energy quality often involve developing a ranking of energy qualities in hierarchical order.