Efficacy is the capacity to produce an effect. It has different specific meanings in different fields. In medicine, it is the ability of an intervention or drug to reproduce a desired effect in expert hands and under ideal circumstances.


In a healthcare context, efficacy indicates the capacity for beneficial change (or therapeutic effect
Therapeutic effect
A therapeutic effect is a consequence of a medical treatment of any kind, the results of which are judged to be desirable and beneficial. This is true whether the result was expected, unexpected, or even an unintended consequence of the treatment...

) of a given intervention (e.g. a medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

, medical device
Medical device
A medical device is a product which is used for medical purposes in patients, in diagnosis, therapy or surgery . Whereas medicinal products achieve their principal action by pharmacological, metabolic or immunological means. Medical devices act by other means like physical, mechanical, thermal,...

, surgical procedure, or a public health
Public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals" . It is concerned with threats to health based on population 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 trial
Randomized controlled trial
A randomized controlled trial is a type of scientific experiment - a form of clinical trial - most commonly used in testing the safety and efficacy or effectiveness of healthcare services or health technologies A randomized controlled trial (RCT) is a type of scientific experiment - a form of...

s, whereas 'pragmatic' trials are used to establish the effectiveness of an intervention.

The concept of 'self-efficacy' is an important one in the self-management of chronic
Chronic (medicine)
A chronic disease is a disease or other human health condition that is persistent or long-lasting in nature. The term chronic is usually applied when the course of the disease lasts for more than three months. Common chronic diseases include asthma, cancer, diabetes and HIV/AIDS.In medicine, the...

 diseases because doctors and patients often do not follow best practice
Best practice
A best practice is a method or technique that has consistently shown results superior to those achieved with other means, and that is used as a benchmark...

 in using a treatment. For instance, a patient using combined oral contraceptive pills to prevent pregnancy may sometimes forget to take a pill at the prescribed time; thus, while the perfect-use failure rate for this form of contraception in the first year of use is just 0.3%, the typical-use failure rate is 8%.http://www.agi-usa.org/pubs/fb_contr_use.html

Self-efficacy is a proximal and direct predictor of intention and of behavior. According to Social Cognitive Theory, a personal sense of control facilitates a change of health behavior. Self-efficacy pertains to a sense of control over one's environment and behavior. Self-efficacy beliefs are cognitions that determine whether health behavior change will be initiated, how much effort will be expended, and how long it will be sustained in the face of obstacles and failures. Self-efficacy influences the effort one puts forth to change risk behavior and the persistence to continue striving despite barriers and setbacks that may undermine motivation. Self-efficacy is directly related to health behavior, but it also affects health behaviors indirectly through its impact on goals. Self-efficacy influences the challenges that people take on as well as how high they set their goals (e.g., "I intend to reduce my smoking," or "I intend to quit smoking altogether"). Individuals with strong self-efficacy select more challenging goals. They focus on opportunities, not on obstacles (e.g., "At my university there is a smoking ban, anyway," instead of "There are still a lot of ashtrays at my university").


In an insolvency context, in particular in relation to an Individual Voluntary Arrangement, the efficacy of such an arrangement is judged by the extent to which it is achieving its aims of getting money in to pay to the creditors of the debtor.


In pharmacology, efficacy, refers to the maximum response achievable from a drug. It is often described by the parameter Emax. Intrinsic activity is a relative term which describes the efficacy relative to a drug which has the highest observed efficacy. Effectiveness refers to the ability of a drug to produce a beneficial effect. A distinction is made between 'method' effectiveness which describes the effect achievable if the drug was taken as prescribed and 'use' effectiveness which is the effect obtained under typical use circumstances when adherence is not 100%. The widely used 'intention to treat' method of analysing clinical trials provides estimates of 'use' effectiveness which are typically biased compared with 'method' effectiveness.


In lighting design, "efficacy" refers to the amount of light (luminous flux
Luminous flux
In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of...

) produced by a lamp (a light bulb or other light source), usually measured in lumens
Lumen (unit)
The lumen is the SI derived unit of luminous flux, a measure of the total "amount" of visible light emitted by a source. Luminous flux differs from power in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux...

, as a ratio of the amount of power consumed to produce it, usually measured in watts. This is not to be confused with efficiency which is always a dimensionless ratio of output divided by input which for lighting relates to the watts of visible power as a fraction of the power consumed in watts. The visible power can be approximated by the area under the Planck curve between 300 nm and 700 nm for a blackbody at the temperature of the filament as a ratio of the total power under the blackbody curve. Efficiency values for light from a heat source are typically less than two percent.


In Lutheranism
Lutheranism is a major branch of Western Christianity that identifies with the theology of Martin Luther, a German reformer. Luther's efforts to reform the theology and practice of the church launched the Protestant Reformation...

 efficacy is used to describe the characteristics of Scripture. The efficacy of Scripture means that it is united with the power of the Holy Spirit and with it, not only demands, but also creates the acceptance of its teaching and that this teaching produces faith and obedience. Efficacy further means that Holy Scripture is not a dead letter, but rather, the power of the Holy Spirit is inherent in it and that Scripture does not compel a mere intellectual assent to its doctrine, resting on logical argumentation, but rather it creates the living agreement of faith. The Smalcald Articles
Smalcald Articles
The Smalcald Articles or Schmalkald Articles are a summary of Lutheran doctrine, written by Martin Luther in 1537 for a meeting of the Schmalkaldic League in preparation for an intended ecumenical Council of the Church.-History:...

 affirm, "in those things which concern the spoken, outward Word, we must firmly hold that God grants His Spirit or grace to no one, except through or with the preceding outward Word." The Formula of Concord
Formula of Concord
Formula of Concord is an authoritative Lutheran statement of faith that, in its two parts , makes up the final section of the Lutheran Corpus Doctrinae or Body of Doctrine, known as...

 teaches that when humans reject the calling of the Holy Spirit, it is not a result of the Word being less efficacious. Instead, contempt for the means of grace is the result of "the perverse will of man, which rejects or perverts the means and instrument of the Holy Ghost, which God offers him through the call, and resists the Holy Ghost, who wishes to be efficacious, and works through the Word
Law and Gospel
In Christianity the relationship between God's Law and the Gospel is a major topic in Lutheran and Reformed theology. In these traditions, the distinction between the doctrines of Law, which demands obedience to God's ethical will, and Gospel, which promises the forgiveness of sins in light of the...


Difference amplifiers

The efficacy of a differential amplifier
Differential amplifier
A differential amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.- Theory :Many electronic devices use differential amplifiers internally....

 is measured by the degree of its rejection of common-mode signals in preference to differential signals. Referred to as common-mode rejection ratio (CMRR); typically specified in decibels.

See also

  • Placebo (origins of technical term)
  • Vaccine efficacy
    Vaccine efficacy
    Vaccine efficacy is defined as the reduction in the incidence of a disease among people who have received a vaccine compared to the incidence in unvaccinated people...

  • Efficiency (disambiguation)
    Efficiency (disambiguation)
    Efficiency is the extent to which time or effort is well used for the intended task or purpose.Efficiency may also refer to:* Efficiency or Studio apartment, a one-room apartment* A measure of goodness for a statistical estimator....

  • Figure of merit
    Figure of merit
    A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application...

  • Self-efficacy
    Self-efficacy is a term used in psychology, roughly corresponding to a person's belief in their own competence.It has been defined as the belief that one is capable of performing in a certain manner to attain certain set of goals. It is believed that our personalized ideas of self-efficacy affect...

  • Effectiveness
    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.-Etymology:...

  • Political efficacy
    Political efficacy
    Political efficacy is a theoretical concept used to explain political behavior in Political Science. It indicates citizens' faith and trust in government and his/her own belief that he/she can understand and influence political affairs. It is commonly measured by surveys and used as an indicator...

  • Potent
    Potent may refer to:*Vair#Potent for the heraldic fur*Warren Potent for the Australian Olympic medalist in shootingSee also:*Potency...

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