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Efficacy Vs Effectiveness

Many persons often get confused with efficacy and Effectiveness.  There is a very minute difference between the two. Firstly, let’s get familiar with both these terms then for better understanding, we will also discuss the difference between efficacy and effectiveness (efficacy Vs effectiveness).

Definition of Efficacy

“Efficacy = best possible results for a particular intervention, under perfect conditions;

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Efficacy is the degree of success to which an intervention does more good than harm under ideal circumstances.

Efficacy studies are frequently conducted in large tertiary care which leans to have more specialized clinicians and better technical equipment than primary care facilities.

Efficacy study conducted in Phase II Trial as the number of patients less (100–300) with specific diseases.”

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Definition of Effectiveness

“Effectiveness = most likely results for a particular intervention under pragmatic or ‘real-life’ conditions (taking into account compliance, dropouts, withdrawals, the learning curve for surgical interventions, etc.) 

Effectiveness assesses whether an intervention does more good than harm when provided under usual circumstances of the healthcare practice.

Therefore, populations in primary care may be a tolerable subject group for effectiveness studies.

Effectiveness studies are conducted in Phase III Trial approx 300–3,000 patients with specific diseases.”

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So, from the above definition, it can be concluded as “How well the treatment/ intervention/ Drug is performed in the small population” is termed as Efficacy and how well the treatment/ intervention/ Drug is performed in real-world is termed as Effectiveness.

When Efficacy study (performed in Phase II Trial) covers a large population then only it will be termed as Effectiveness studies (performed in Phase III Trial) as now it will be closer to the real world.

Efficacy Vs Effectiveness

“Efficacy is the effect in perfect conditions whereas effectiveness is the effect in the real world.

When a study assesses efficacy, it is looking at whether the drug given in the precise manner described in the study is able to influence an outcome of interest (e.g. tumor size) in the selected population (e.g. cancer patients with no other ongoing diseases).

When a study is assessing effectiveness, it is determining whether a treatment will influence the disease. In an effectiveness study, it is necessary that patients are treated as they would be when the treatment is prescribed indefinite practice.

That would mean that there should be no aspects of the study designed to increase patient compliance above those that would occur in routine clinical practice.

The outcomes in effectiveness studies are also easily applicable than in most efficacy studies (for example does the patient feel better, come to the hospital less or live longer in effectiveness studies as disparate to better test scores or lower cell counts in efficacy studies).

There is usually less stiff control of the type of patient to be included in the effectiveness studies than in efficacy studies, as the researchers are interested in whether the drug will have a broad effect in the population of the patients with the disease.”


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