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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 produce a desired effect.
In medicine, efficacy indicates the capacity for beneficial change (or therapeutic effect) of a given intervention (e.g. 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.
In pharmacology, efficacy (Emax) refers to the maximum response achievable from a drug. Intrinsic activity is a relative term that describes a drug's efficacy relative to a drug with 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. Efficacy is the magnitude of response with respect to the Kd.
In lighting design, "efficacy" refers to the amount of light (luminous flux) produced by a lamp (a lamp or other light source), usually measured in lumens, 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 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 Reformed Theology (esp. in Lutheran but also in Calvinist doctrine) efficacy is an attribute 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 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 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..."
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