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 | 一般醫學 證據醫學 護理學 | 藥學 期刊點數 綜合 | 院外連結 | Levels of EBM  | 

 

Study Designs
 

Case-Control Studies

Advantages:

  • quick and cheap;
  • only feasible method for very rare disorders or those with long lag between exposure and outcome;
  • fewer subjects needed than cross-sectional studies.

Disadvantages:

  • reliance on recall or records to determine exposure status;
  • confounders;
  • selection of control groups is difficult;
  • potential bias: recall, selection.

Cross-Sectional Survey

Advantages:

  • cheap and simple;
  • ethically safe.

Disadvantages:

  • establishes association at most, not causality;
  • recall bias susceptibility;
  • confounders may be unequally distributed;
  • Neyman bias;
  • group sizes may be unequal.

Cohort Study

Advantages:

  • ethically safe;
  • subjects can be matched;
  • can establish timing and directionality of events;
  • eligibility criteria and outcome assessments can be standardised;
  • administratively easier and cheaper than RCT.

Disadvantages:

  • controls may be difficult to identify;
  • exposure may be linked to a hidden confounder;
  • blinding is difficult;
  • randomisation not present;
  • for rare disease, large sample sizes or long follow-up necessary.

Randomised Controlled Trial

Advantages:

  • unbiased distribution of confounders;
  • blinding more likely;
  • randomisation facilitates statistical analysis.

Disadvantages:

  • expensive: time and money;
  • volunteer bias;
  • ethically problematic at times.

Crossover Design

Advantages:

  • all subjects serve as own controls and error variance is reduced thus reducing sample size needed;
  • all subjects receive treatment (at least some of the time);
  • statistical tests assuming randomisation can be used;
  • blinding can be maintained.

Disadvantages:

  • all subjects receive placebo or alternative treatment at some point;
  • washout period lengthy or unknown;
  • cannot be used for treatments with permanent effects

 

 
Levels of Evidence
 

Oxford Centre for Evidence-based Medicine Levels of Evidence (May 2001) 

Level

Therapy/Prevention, Aetiology/Harm

Prognosis

Diagnosis

Differential diagnosis/symptom prevalence study

Economic and decision analyses

1a

SR (with homogeneity*) of RCTs

SR (with homogeneity*) of inception cohort studies; CDR† validated in different populations

SR (with homogeneity*) of Level 1 diagnostic studies; CDR† with 1b studies from different clinical centres

SR (with homogeneity*) of prospective cohort studies

SR (with homogeneity*) of Level 1 economic studies

1b

Individual RCT (with narrow Confidence Interval‡)

Individual inception cohort study with > 80% follow-up; CDR† validated in a single population

Validating** cohort study with good††† reference standards; or CDR† tested within one clinical centre

Prospective cohort study with good follow-up****

Analysis based on clinically sensible costs or alternatives; systematic review(s) of the evidence; and including multi-way sensitivity analyses

1c

All or none§

All or none case-series

Absolute SpPins and SnNouts††

All or none case-series

Absolute better-value or worse-value analyses ††††

2a

SR (with homogeneity*) of cohort studies

SR (with homogeneity*) of either retrospective cohort studies or untreated control groups in RCTs

SR (with homogeneity*) of Level >2 diagnostic studies

SR (with homogeneity*) of 2b and better studies

SR (with homogeneity*) of Level >2 economic studies

2b

Individual cohort study (including low quality RCT; e.g., <80% follow-up)

Retrospective cohort study or follow-up of untreated control patients in an RCT; Derivation of CDR† or validated on split-sample§§§ only

Exploratory** cohort study with good†††reference standards; CDR† after derivation, or validated only on split-sample§§§ or databases

Retrospective cohort study, or poor follow-up

Analysis based on clinically sensible costs or alternatives; limited review(s) of the evidence, or single studies; and including multi-way sensitivity analyses

2c

"Outcomes" Research; Ecological studies

"Outcomes" Research

 

Ecological studies

Audit or outcomes research

3a

SR (with homogeneity*) of case-control studies

 

SR (with homogeneity*) of 3b and better studies

SR (with homogeneity*) of 3b and better studies

SR (with homogeneity*) of 3b and better studies

3b

Individual Case-Control Study

 

Non-consecutive study; or without consistently applied reference standards

Non-consecutive cohort study, or very limited population

Analysis based on limited alternatives or costs, poor quality estimates of data, but including sensitivity analyses incorporating clinically sensible variations.

4

Case-series (and poor quality cohort and case-control studies§§)

Case-series (and poor quality prognostic cohort studies***)

Case-control study, poor or non-independent reference standard

Case-series or superseded reference standards

Analysis with no sensitivity analysis

5

Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"

Expert opinion without explicit critical appraisal, or based on economic theory or "first principles"

Produced by Bob Phillips, Chris Ball, Dave Sackett, Doug Badenoch, Sharon Straus, Brian Haynes, Martin Dawes since November 1998.

 Notes

Users can add a minus-sign "-" to denote the level of that fails to provide a conclusive answer because of:

·          EITHER a single result with a wide Confidence Interval (such that, for example, an ARR in an RCT is not statistically significant but whose confidence intervals fail to exclude clinically important benefit or harm)

·          OR a Systematic Review with troublesome (and statistically significant) heterogeneity.

Such evidence is inconclusive, and therefore can only generate Grade D recommendations.  

PRIVATE*

By homogeneity we mean a systematic review that is free of worrisome variations (heterogeneity) in the directions and degrees of results between individual studies. Not all systematic reviews with statistically significant heterogeneity need be worrisome, and not all worrisome heterogeneity need be statistically significant. As noted above, studies displaying worrisome heterogeneity should be tagged with a "-" at the end of their designated level.

Clinical Decision Rule. (These are algorithms or scoring systems which lead to a prognostic estimation or a diagnostic category. )

See note #2 for advice on how to understand, rate and use trials or other studies with wide confidence intervals.

§

Met when all patients died before the Rx became available, but some now survive on it; or when some patients died before the Rx became available, but none now die on it.

§§

By poor quality cohort study we mean one that failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same (preferably blinded), objective way in both exposed and non-exposed individuals and/or failed to identify or appropriately control known confounders and/or failed to carry out a sufficiently long and complete follow-up of patients. By poor quality case-control study we mean one that failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same (preferably blinded), objective way in both cases and controls and/or failed to identify or appropriately control known confounders.

§§§

Split-sample validation is achieved by collecting all the information in a single tranche, then artificially dividing this into "derivation" and "validation" samples.

††

An "Absolute SpPin" is a diagnostic finding whose Specificity is so high that a Positive result rules-in the diagnosis. An "Absolute SnNout" is a diagnostic finding whose Sensitivity is so high that a Negative result rules-out the diagnosis.

‡‡

Good, better, bad and worse refer to the comparisons between treatments in terms of their clinical risks and benefits.

†††

Good reference standards are independent of the test, and applied blindly or objectively to applied to all patients. Poor reference standards are haphazardly applied, but still independent of the test. Use of a non-independent reference standard (where the 'test' is included in the 'reference', or where the 'testing' affects the 'reference') implies a level 4 study.

††††

Better-value treatments are clearly as good but cheaper, or better at the same or reduced cost. Worse-value treatments are as good and more expensive, or worse and the equally or more expensive.

**

Validating studies test the quality of a specific diagnostic test, based on prior evidence. An exploratory study collects information and trawls the data (e.g. using a regression analysis) to find which factors are 'significant'.

***

By poor quality prognostic cohort study we mean one in which sampling was biased in favour of patients who already had the target outcome, or the measurement of outcomes was accomplished in <80% of study patients, or outcomes were determined in an unblinded, non-objective way, or there was no correction for confounding factors.

****

Good follow-up in a differential diagnosis study is >80%, with adequate time for alternative diagnoses to emerge (eg 1-6 months acute, 1 - 5 years chronic)

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 which has potentially clinically important differences than the original study situation.

 

Critical Appraisal
 

 

Finding the Best Evidence
 
  1. Identify terms to fit your PICO question

  2. Look for secondary sources

  3. Search for primary sources

 

Identify terms to fit your PICO question
 
Patient Intervention Comparison Outcome
       

 

Search for Primary Sources
 
Use methodological filters to target the right type of study:

PubMED filters for:

  • therapy
  • diagnosis
  • prognosis
  • aetiology
 

 

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