Evidence-based medicine
Evidence-based medicine (EBM) aims to apply evidence gained from the scientific method to certain parts of medical practice. It seeks to assess the quality of evidence relevant to the risks and benefits of treatments (including lack of treatment). According to the Centre for Evidence-Based Medicine, "Evidence-based medicine is the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients."
EBM recognizes that many aspects of medical care depend on individual factors such as quality and value-of-life judgments, which are only partially subject to scientific methods. EBM, however, seeks to clarify those parts of medical practice that are in principle subject to scientific methods and to apply these methods to ensure the best prediction of outcomes in medical treatment, even as debate about which outcomes are desirable continues.
Practicing evidence-based medicine requires clinical expertise, but also expertise in retrieving, interpreting, and applying the results of scientific studies and in communicating the risks and benefits of different courses of action to patients.

# Overview
Using techniques from science, engineering, and statistics, such as meta-analysis of medical literature, risk-benefit analysis, and randomized controlled trials, EBM aims for the ideal that healthcare professionals should make "conscientious, explicit, and judicious use of current best evidence" in their everyday practice.
Generally, there are three distinct, but interdependent, areas of EBM. The first is to treat individual patients with acute or chronic pathologies by treatments supported in the most scientifically valid medical literature. Thus, medical practitioners would select treatment options for specific cases based on the best research for each patient they treat. The second area is the systematic review of medical literature to evaluate the best studies on specific topics. This process can be very human-centered, as in a journal club, or highly technical, using computer programs and information techniques such as data mining. Increased use of information technology turns large volumes of information into practical guides. Finally, evidence-based medicine can be understood as a medical "movement" in which advocates work to popularize the method and usefulness of the practice in the public, patient communities, educational institutions, and continuing education of practicing professionals.
Evidence-based medicine has demoted ex cathedra statements of the "medical expert" to the least valid form of evidence. All "experts" are now expected to reference their pronouncements to scientific studies.

# Classification
Two types of evidence-based medicine have been proposed.

## Evidence-based guidelines
Evidence-based guidelines (EBG) is the practice of evidence-based medicine at the organizational or institutional level. This includes the production of guidelines, policy, and regulations. This approach has also been called evidence based healthcare.

## Evidence-based individual decision making
Evidence-based individual decision (EBID) making is evidence-based medicine as practiced by the individual health care provider. There is concern that current evidence-based medicine focuses excessively on EBID.

# History
Although testing medical interventions for efficacy has existed since the time of Avicenna's The Canon of Medicine in the 11th century, it was only in the 20th century that this effort evolved to impact almost all fields of health care and policy. Professor Archie Cochrane, a Scottish epidemiologist, through his book Effectiveness and Efficiency: Random Reflections on Health Services (1972) and subsequent advocacy, caused increasing acceptance of the concepts behind evidence-based practice. Cochrane's work was honoured through the naming of centres of evidence-based medical research  Cochrane Centres  and an international organization, the Cochrane Collaboration. The explicit methodologies used to determine "best evidence" were largely established by the McMaster University research group led by David Sackett and Gordon Guyatt. The term "evidence based" was first used in 1990 by David Eddy. The term "evidence-based medicine" first appeared in the medical literature in 1992 in a paper by Guyatt et al.

# Qualification of evidence
Evidence-based medicine categorizes different types of clinical evidence and ranks them according to the strength of their freedom from the various biases that beset medical research. For example, the strongest evidence for therapeutic interventions is provided by systematic review of randomized, double-blind, placebo-controlled trials involving a homogeneous patient population and medical condition. In contrast, patient testimonials, case reports, and even expert opinion have little value as proof because of the placebo effect, the biases inherent in observation and reporting of cases, difficulties in ascertaining who is an expert, and more.
Systems to stratify evidence by quality have been developed, such as this one by the U.S. Preventive Services Task Force   for ranking evidence about the effectiveness of treatments or screening:
- Level I: Evidence obtained from at least one properly designed randomized controlled trial.
- Level II-1: Evidence obtained from well-designed controlled trials without randomization.
- Level II-2: Evidence obtained from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
- Level II-3: Evidence obtained from multiple time series with or without the intervention. Dramatic results in uncontrolled trials might also be regarded as this type of evidence.
- Level III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.
The UK National Health Service uses a similar system with categories labeled A, B, C, and D. The above Levels are only appropriate for treatment or interventions; different types of research are required for assessing diagnostic accuracy or natural history and prognosis, and hence different "levels" are required. For example, the Oxford Centre for Evidence-based Medicine suggests levels of evidence (LOE) according to the study designs and critical appraisal of prevention, diagnosis, prognosis, therapy, and harm studies:
- Level A: Consistent Randomised Controlled Clinical Trial, cohort study, all or none, clinical decision rule validated in different populations.
- Level B: Consistent Retrospective Cohort, Exploratory Cohort, Ecological Study, Outcomes Research, case-control study; or extrapolations from level A studies.
- Level C: Case-series study or extrapolations from level B studies.
- Level D: Expert opinion without explicit critical appraisal, or based on physiology, bench research or first principles.
A newer system is by the Grade Working Group and takes in account more dimensions that just the quality of medical evidence.
"Extrapolations" are where data is used in a situation which has potentially clinically important differences than the original study situation. Thus, the quality of evidence to support a clinical decision is a combination of the quality of research data and the clinical 'directness' of the data.
Despite the differences between systems, the purposes are the same: to guide users of clinical research information about which studies are likely to be most valid. However, the individual studies still require careful critical appraisal.

## Categories of recommendations
In guidelines and other publications, recommendation for a clinical service is classified by the balance of risk versus benefit of the service and the level of evidence on which this information is based. The U.S. Preventive Services Task Force uses:
- Level A: Good scientific evidence suggests that the benefits of the clinical service substantially outweighs the potential risks. Clinicians should discuss the service with eligible patients.
- Level B: At least fair scientific evidence suggests that the benefits of the clinical service outweighs the potential risks. Clinicians should discuss the service with eligible patients.
- Level C: At least fair scientific evidence suggests that there are benefits provided by the clinical service, but the balance between benefits and risks are too close for making general recommendations. Clinicians need not offer it unless there are individual considerations.
- Level D: At least fair scientific evidence suggests that the risks of the clinical service outweighs potential benefits. Clinicians should not routinely offer the service to asymptomatic patients.
- Level I: Scientific evidence is lacking, of poor quality, or conflicting, such that the risk versus benefit balance cannot be assessed. Clinicians should help patients understand the uncertainty surrounding the clinical service.

# Statistical measures in evidence-based medicine
Evidence-based medicine attempts to express clinical benefits of tests and treatments using mathematical methods. Tools used by practitioners of evidence-based medicine include:
- Likelihood ratios. The pretest probability of a particular diagnosis, multiplied by the likelihood ratio, determines the posttest probability. This reflects Bayes' theorem. The differences in likelihood ratio between clinical tests can be used to prioritize clinical tests according to their usefulness in a given clinical situation.
- The area under the receiver operator characteristic curve (AUC-ROC) reflects the relationship between sensitivity and specificity for a given test. High-quality tests will have an AUC-ROC approaching 1, and high-quality publications about clinical tests will provide information about the AUC-ROC. Cutoff values for positive and negative tests can influence specificity and sensitivity, but they do not affect AUC-ROC.
- Number needed to treat or Number needed to harm are ways of expressing the effectiveness and safety of an intervention in a way that is clinically meaningful. In general, NNT is always computed with respect to two treatments A and B, with A typically a drug and B a placebo (in our example above, A is a 5-year treatment with the hypothetical drug, and B is no treatment). A defined endpoint has to be specified (in our example: the appearance of colon cancer in the 5 year period). If the probabilities pA and pB of this endpoint under treatments A and B, respectively, are known, then the NNT is computed as 1/(pB-pA). The NNT for breast mammography is 1/285, so 285 mammograms need to be performed to diagnose one breast cancer. As another example, an NNT of 4 means if 4 patients are treated, only one would respond.
An NNT of 1 is the most effective and means each patient treated responds, e.g., in comparing antibiotics with placebo in the eradication of Helicobacter pylori. An NNT of 2 or 3 indicates that a treatment is quite effective (with one patient in 2 or 3 responding to the treatment). An NNT of 20 to 40 can still be considered clinically effective.

# Quality of assessment of Risk of Bias (RoB) in studies
Frameworks to assess RoB in clinical studies, depending on the study design, are available at  / and /

## Quality of clinical trial publications
Evidence-based medicine attempts to objectively evaluate the quality of clinical research by critically assessing techniques reported by researchers in their publications.
- Trial design considerations. High-quality studies have clearly-defined eligibility criteria, and have minimal missing data.
- Generalizability considerations. Studies may only be applicable to narrowly-defined patient populations, and may not be generalizable to clinical practice.
- Followup. Sufficient time for defined outcomes to occur can influence the study outcomes and the statistical power of a study to detect differences between a treatment and control arm.
- Power. This is the number of patients enrolled in the trial. A complex mathematical calculation can determine if the number of patients is sufficient to detect a difference between treatment arms. A negative study may reflect a lack of benefit, or simply a lack of sufficient quantities of patients to detect a difference.

## Limitations of available evidence
It is recognised that not all evidence is made accessible, that this can limit the effectiveness of any approach, and that effort to reduce various publication and retrieval biases is required.
Failure to publish negative trials is the most obvious gap, and moves to register all trials at the outset, and then to pursue their results, are underway. Changes in publication methods, particularly related to the Web, should reduce the difficulty of obtaining publication for a paper on a trial that concludes it did not prove anything new, including its starting hypothesis.
Treatment effectiveness reported from clinical studies may be higher than that achieved in later routine clinical practice due to the closer patient monitoring during trials that leads to much higher compliance rates.

# Effectiveness
There are mixed reports about whether evidence-based medicine is effective.
Efficacy is how well a treatment performs under ideal circumstances (and is studied in a clinical trial) whereas effectiveness is performance in the real world.
Using the classification scheme above --dividing evidence-based medicine into evidence-based guidelines (EBG) and evidence-based individual decision (EBID)-- may explain the conflict. It is difficult to find evidence that EBID improves health care, whereas there is growing evidence of improvements in the efficacy of health care when evidence-based medicine is practiced at the organizational level.  One of the virtues of healthcare accreditation is that it offers an opportunity to assess the overall functioning of a hospital or healthcare organisation against the best of the currently-available evidence and to assist the hospital or healthcare organisation to move towards a more effective application of evidence-based medical.
Effectiveness can be measured with the RE-AIM (reach, effectiveness, adoption, implementation, and maintenance) framework.

# Criticism of evidence-based medicine
Critics of EBM say lack of evidence and lack of benefit are not the same, and that the more data are pooled and aggregated, the more difficult it is to compare the patients in the studies with the patient in front of the doctor  that is, EBM applies to populations, not necessarily to individuals. In The limits of evidence-based medicine, Tonelli argues that "the knowledge gained from clinical research does not directly answer the primary clinical question of what is best for the patient at hand."  Tonelli suggests that proponents of evidence-based medicine discount the value of clinical experience.
However, many proponents of EBM argue that the best practice of EBM does not discount clinicians' own experience. For instance David Sackett writes that "the practice of evidence based medicine means integrating individual clinical expertise with the best available external clinical evidence from systematic research".
- Although evidence-based medicine is becoming regarded as the "gold standard" for clinical practice and treatment guidelines, there are a number of reasons why most current medical and surgical practices do not have a strong literature base supporting them.
- In some cases, such as in open-heart surgery, conducting randomized, placebo-controlled trials would be unethical, although observational studies may address these problems to some degree.
- Certain groups have been historically under-researched (racial minorities and people with many co-morbid diseases), and thus the literature is sparse in areas that do not allow for generalizing.
- The types of trials considered "gold standard" (i.e. randomized double-blind placebo-controlled trials) may be expensive, so that funding sources play a role in what gets investigated. For example, public authorities may tend to fund preventive medicine studies to improve public health as a whole, while pharmaceutical companies fund studies intended to demonstrate the efficacy and safety of particular drugs.
- The studies that are published in medical journals may not be representative of all the studies that are completed on a given topic (published and unpublished) or may be misleading due to conflicts of interest (i.e. publication bias). Thus the array of evidence available on particular therapies may not be well-represented in the literature. A 2004 statement by the International Committee of Medical Journal Editors that they will refuse to publish clinical trial results if the trial was not recorded publicly at its outset, may help with this, although this has to date still not been actioned.
- The quality of studies performed varies, making it difficult to generalize about the results.
An additional problem is that large randomized controlled trials are useful for examining discrete interventions for carefully defined medical conditions. The more complex the patient population (e.g. severity of condition, co-morbid conditions, etc) in the study, the more difficult it is to assess the treatment effect (i.e., treatment mean - control group mean), relative to the random variation (within group variation of both the treatment and control groups). Because of this, a number of studies obtain non-significant results, either because there is insufficient power to show a difference, or because the groups are not well-enough "controlled". Ironically, the fewer restrictions there are on who can participate in a study (i.e., the greater the generalizability of the results to the type of patient being seen in a real world setting) the less able the study to detect real differences between groups for a given sample size.
Furthermore, evidence-based guidelines do not remove the problem of extrapolation to different populations or longer timeframes. Even if several top-quality studies are available, questions always remain about how far, and to which populations, their results are "generalizable".Empty citation (help).mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em} Furthermore, skepticism about results may always be extended to areas not explicitly covered: for example a drug may influence a "secondary endpoint" such as test result (blood pressure, glucose, or cholesterol levels) without having the power to show that it decreases overall mortality or morbidity in a population.
In managed healthcare systems, evidence-based guidelines have been used as a basis for denying insurance coverage for some treatments which are held by the physicians involved to be effective, but of which randomized controlled trials have not yet been published. In some cases, these denials were based upon questions of induction and efficacy as discussed above. For example, if an older generic statin drug has been shown to reduce mortality, is this enough evidence for use of a much more expensive newer statin drug which lowers cholesterol more effectively, but for which mortality reductions have not had time enough to be shown?  If a new, costly therapy that works on tumor blood vessels causes two kinds of cancer to go into remission, is it justified as an expense in a third kind of cancer, before this has specifically been proven?  . Kaiser Permanente did not change its methods of evaluating whether or not new therapies were too "experimental" to be covered, until it was successfully sued twice: once for delaying IVF treatments for two years after the courts determined that scientific evidence of efficacy and safety had reached the "reasonable" stage, and in another case where Kaiser refused to pay for liver transplantation in infants when it had already been shown to be effective in adults, on the basis that use in infants was still "experimental."  Here again the problem of induction plays a key role in arguments.

## The steps of EBM are impractical
"Information mastery" of secondary publications is advocated as a practical alternative

# Evidence-Based Practice
In his 1996 inaugural speech as President of the Royal Statistical Society, Adrian Smith held out evidence-based medicine as an exemplar for all public policy. He proposed that Evidence-Based Practice should be established for education, prisons and policing policy and all areas of government.