Efficiency in general describes the extent to which time, effort or cost is well used for the intended task or purpose. It is often used with the specific purpose of relaying the capability of a specific application of effort to produce a specific outcome effectively with a minimum amount or quantity of waste, expense, or unnecessary effort. "Efficiency" has widely varying meanings in different disciplines.
The term "efficient" is very much confused and misused with the term "effective". In general, efficiency is a measurable concept, quantitatively determined by the ratio of output to input. "Effectiveness", is a relatively vague, non-quantitative concept, mainly concerned with achieving objectives. In several of these cases, efficiency can be expressed as a result as percentage of what ideally could be expected, hence with 100% as ideal case. This does not always apply, not even in all cases where efficiency can be assigned a numerical value, e.g. not for specific impulse.
A simple way of distinguishing between efficiency and effectiveness is the saying, "Efficiency is doing things right, while Effectiveness is doing the right things." This is based on the premise that selection of objectives of a process are just as important as the quality of that process.
A slightly broader mode of efficiency that nevertheless remains consistent with the "percentage" definition in many cases is to say that efficiency corresponds to the ratio r=P/C of the amount P of some valuable resource produced, per amount C of valuable resources consumed. This may correspond to a percentage if products and consumables are quantified in compatible units, and if consumables are transformed into products via a conservative process. For example, in the analysis of the energy conversion efficiency of heat engines in thermodynamics, the product P may be the amount of useful work output, while the consumable C is the amount of high-temperature heat input. Due to the conservation of energy, P can never be greater than C, and so the efficiency r is never greater than 100% (and in fact must be even less at finite temperatures).
In science and technology 
In physics and engineering 
- Useful work per quantity of energy, mechanical advantage over ideal mechanical advantage, often denoted by the Greek lowercase letter η (Eta):
- Efficient energy use, the objective of maximising efficiency
- Lift-to-drag ratio
- Faraday efficiency, electrolysis
- Quantum efficiency, a measure of sensitivity of a photosensitive device
- Grating efficiency, a generalization of the reflectance of a mirror, extended to a diffraction grating
In economics 
- Productivity improving technologies (historical)
- Economic efficiency, a general term, capturing the amount of waste or other undesirable features
- Market efficiency is the extent to which a given market resembles the ideal of an efficient market
- Pareto efficiency, making one individual better off, without making any other individual worse off
- Kaldor-Hicks efficiency, like a less stringent Pareto efficiency
- Allocative efficiency, an optimal distribution of goods
- Efficiency wages, paying workers more than the market rate for increased productivity
- Business efficiency, expenses as a percentage of revenue, etc.
- Efficiency Movement, of the Progressive Era (1890–1932), advocated efficiency in the economy, society and government
In other sciences 
- Efficiency (statistics), a count of desirability of an estimator
- Material efficiency, compares material requirements between construction projects or physical processes
- Administrative efficiency, increasing transparency within public authorities and simplification of rules and procedures for citizens and businesses
See also 
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