Evidence Based Medicine: Resources-

  1. Evidence-Based Medicine (EBM)Resources: Dartmouth Biomedical Libraries
  2. Oxford Centre for Evidence-based Medicine – Levels of Evidence (March 2009)
  3. Evidence based medicine: an approach to clinical problem-solving-William Rosenberg, Anna Donald
  4. Evidence-Based Medicine: Wikipedia

Definition:  Evidence Based Medicine, is a process of turning clinical problems into questions and then systematically locating, appraising, and using contemporaneous research findings as the basis for clinical decisions.

“Evidence-based medicine is a set of principles and methods intended to ensure that to the greatest extent possible, medical decisions, guidelines, and other types of policies are based on and consistent with good evidence of effectiveness and benefit.” – David M. Eddy(2005).

Evidence based medicine is about asking questions, finding and appraising the relevant data, and harnessing that information for everyday clinical practice.

Steps: 5 steps in Evidence based medicine-

  1. Setting the Question-formulating a clear clinical question from a patient’s problem includes critical questioning, study design and levels of evidence.
  2. Systemic retrieval of best evidence– searching the literature for relevant articles.
  3. Critical appraisal of evidence-evaluating the evidence for its
    validity and usefulness in the following aspects:a.Systematic errors due to selection bias, information bias and confounding.
    b.Quantitative aspects of diagnosis and treatment.c.The effect size and aspects regarding its precision.d.Clinical importance of results.e.External validity or generalizability.
  4. Application of results in practice– implementing useful findings in clinical practice.
  5. Evaluation of final outcome after implementation of results.

LEVELS OF EVIDENCE: (based on Oxford Centre for Evidence-based Medicine)– for assessing quality of evidence.

  1. Level 1: Randomized Controlled Trial(RCT)-
    1a. Systemic Review (with homogeneity) of RCTs
    1b. Individual RCT (with narrow ‘Confidence Interval’)
    1c. All or none RCT
  2. Level 2-2a. Systemic Review (with homogeneity) of cohort studies
    2b. Individual cohort study (including low quality RCT; e.g., <80% follow-up)
    2c. “Outcomes” Research; Ecological studies
  3. Level 3– 3a. Systemic Review (with homogeneity) of case-control studies.
    3b. Individual Case-Control Study.
  4. Level 4– Case-series (and poor quality cohort and case-control studies)
  5. Level 5– Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”.

Evidence based medicine

Levels of Evidence

Grades of Recommendation:

A consistent Level 1 studies


B consistent Level 2 or 3 studies or extrapolations* from Level 1 studies


C Level 4 studies or extrapolations from Level 2 or 3 studies
D Level 5 evidence or troublingly inconsistent or inconclusive studies of any level

“Extrapolations” are where data is used in a situation that has potentially clinically important differences than the original study situation.

After assessing all the best evidence, treatment may be categorized as-
(1) beneficial,
(2) harmful, or
(3) evidence did not support either benefit or harm.


  • Level A: Good scientific evidence – benefits of clinical service >> potential risks→ Clinicians should discuss the service with eligible patients.
  • Level B: Fair scientific evidence- benefits of clinical service >  potential risks→ Clinicians should discuss the service with eligible patients.
  • Level C: Fair scientific evidence- benefits provided by the clinical service→ the balance between benefits and risks are too close→  Clinicians need not offer  for general recommendation except for individual considerations.
  • Level D: Fair scientific evidence→ risks of clinical service >  potential benefits→ Clinicians should not routinely offer the service.
  • Level I: Scientific evidence- lacking/poor quality/conflicting→ risk vs benefit balance cannot be assessed→ Clinicians should help patients understand the uncertainty surrounding the clinical service.

Statistical measures:

Tools used by practitioners of evidence-based medicine are:

  • Likelihood ratio – The pre-test odds of a particular diagnosis, multiplied by the likelihood ratio, determines the post-test odds. (Odds can be calculated from, and then converted to the 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.
  • AUC-ROC ( Area under the receiver operating characteristic curve)-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. Cut-off values for positive and negative tests can influence specificity and sensitivity, but they do not affect AUC-ROC.
  • NNT (Number needed to treat / harm)- Number needed to treat or Number needed to harm are ways of expressing the effectiveness and safety of interventions in a way that is clinically meaningful. NNT is the number of people who need to be treated in order to achieve the desired outcome in one patient.

ADVANTAGES of Evidence Based Medicine:

  • Integration medical education with clinical
  • It can be learnt by people from different backgrounds & at any stage in their careers.
  • It has potential for improving continuity and uniformity of care through the common approaches and guidelines developed by its practitioners.
  • Better use of limited resources by evaluating clinical effectiveness of treatments and services.
  • For individuals-
    * Enables clinicians to upgrade their knowledge base
    * Improves clinicians’ understanding of research
    methods and makes them more critical in using data
    * Improves confidence in management decisions
    * Improves computer literacy and data searching
    * Improves reading habits
  • For clinical teams-
    * Gives team a framework for group problem solving
    and for teaching.
    * Enables juniors to contribute usefully to team.
  • For patients-
    * More effective use of resources.
    * Better communication with patients about the rationale behind management decisions.


  • It takes time both to learn and to practice.
  • Establishing the infrastructure for practicing evidence based medicine costs money esp. for computer hardware & software and online paid resources( e.g. Clinical Articles).
  • It exposes gaps in evidence- may be frustrating for new young clinicians.
  • Authoritarian clinicians may see evidence based medicine as a threat; it may cause them to lose face by sometimes exposing their current practice as obsolete or occasionally even dangerous→ may alter the dynamics of the team, removing hierarchical distinctions that are based on seniority.