Efficiency is the capability of acting or producing effectively with a minimum of waste expense or unnecessary effort. The term has widely variant meanings in different disciplines. See
Effectiveness means the capability of or success in achieving a given goal. Contrary to efficiencythe focus of effectiveness is the achievement as such not the resources spent so not anything that is effective has to be efficient but anything that is efficient also has to be effective.
Efficiency is the (often measurable) ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without waste.[1][2][3][4][5] In more mathematical or scientific terms, it is a measure of the extent to which input is well used for an intended task or function (output). It often specifically comprises the capability of a specific application of effort to produce a specific outcome with a minimum amount or quantity of waste, expense, or unnecessary effort. Efficiency of course refers to very different inputs and outputs in different fields and industries.
Efficiency is very often confused with effectiveness. In general, efficiency is a measurable concept, quantitatively determined by the ratio of useful output to total input. Effectiveness is the simpler concept of being able to achieve a desired result, which can be expressed quantitatively but doesn't usually require more complicated mathematics than addition. Efficiency can often be expressed as a percentage of the result that could ideally be expected, for example if no energy were lost due to friction or other causes, in which case 100% of fuel or other input would be used to produce the desired result. This does not always apply, not even in all cases in which efficiency can be assigned a numerical value, e.g. not for specific impulse.
Efficacy, efficiency, and effectivity are terms that can, in some cases, be interchangeable with the term effectiveness. The word effective is sometimes used in a quantitative way, "being very effective or not very effective". However, neither effectiveness, nor effectively, inform about the direction (positive or negative) and the comparison to a standard of the given effect. Efficacy, on the other hand, is the extent to which a desired effect is achieved; the ability to produce a desired amount of the desired effect, or the success in achieving a given goal. Contrary to the term efficiency, the focus of efficacy is the achievement as such, not the resources spent in achieving the desired effect. Therefore, what is effective is not necessarily efficacious, and what is efficacious is not necessarily efficient.
In medicine, efficacy is the capacity for beneficial change (or therapeutic effect) of a given intervention (for example a drug, medical device, surgical procedure, or a public health intervention). If efficacy is established, an intervention is likely to be at least as good as other available interventions, to which it will have been compared. Comparisons of this type are typically made in 'explanatory' randomized controlled trials, whereas 'pragmatic' trials are used to establish the effectiveness of an intervention.
When talking in terms of efficacy vs. effectiveness, effectiveness relates to how well a treatment works in the practice of medicine, as opposed to efficacy, which measures how well treatment works in clinical trials or laboratory studies.