Effectiveness is the capability of producing a desired result. When something is deemed effective, it means it has an intended or expected outcome, or produces a deep, vivid impression.
The origin of the word "effective" stems from the Latin word effectīvus, which means creative, productive or effective. It surfaced in Middle English between 1300–1400 A.D.
In mathematics, effective is sometimes used as a synonym of algorithmically computable.
In physics, an effective theory is, similar to a phenomenological theory, a framework intended to explain certain (observed) effects without the claim that the theory correctly models the underlying (unobserved) processes.
In heat transfer, effectiveness is a measure of the performance of a heat exchanger when using the NTU method.
In medicine, effectiveness relates to how well a treatment works in practice, as opposed to efficacy, which measures how well it works in clinical trials or laboratory studies.
In management, effectiveness relates to getting the right things done. Peter Drucker reminds us that "effectiveness can and must be learned".
In human–computer interaction, effectiveness is defined as "the accuracy and completeness of users' tasks while using a system".
In military science, effectiveness is a criterion used to assess changes determined in the target system, in its behavior, capability, or assets, tied to the attainment of an end state, achievement of an objective, or creation of an effect.
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.
Other synonyms for effectiveness include: clout, capability, success, weight, performance. Antonyms for effectiveness include: uselessness, ineffectiveness.
Another source of confusion regarding the term effectiveness is its relationship with the term affectiveness. Due to the similarity in the way these two words are spelled, they are often confused with each other and used incorrectly. The term affectiveness (noun) is derived from the root word affective (adjective) meaning concerned with arousing emotions or affection or relating to moods, attitudes or feelings. Simply stated, effective means outcome and efficiency means output.