Types of Evidence in Medicine

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Lesson Plan

Aims and Objectives:

  • To define evidence-based medicine and outlay its importance in medical practice
  • To describe the scientific method of research
  • To list and describe the types of evidence available and rank them according to the strength

Evidence-based Medicine[edit]

"Evidence-based medicine is the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients."[1]It aims to apply evidence gained from the scientific method to certain parts of medical practice. It seeks to assess the quality of evidence[2] relevant to the risks and benefits of treatments (including lack of treatment).

The importance of scientific evidence is best observed when previous literature, medical rationale, patient preferences and practitioner experience are combined.[3] This combination intends to provide the patient with the best known standard of care. The main drawback for evidence-based medicine[4] is the fact that studies of populations are not necessarily relevant when a medical practitioner is treating a single patient. Despite this, evidence-based medicine still remains the gold-standard for analyzing the risks and benefits of certain treatments.[5]

The Scientific Method[edit]

The scientific method is the method of choice when gathering medical evidence. It follows a strict structure to ensure that evidence gathering is standardized, regardless of the type of evidence gathering used. Its goal is to test a given hypothesis (theory).

The process is as follows:[6]

  1. Define the question
  2. Gather information and resources
  3. Form hypothesis
  4. Perform experiment and collect data
  5. Analyze data
  6. Interpret data and draw conclusions
  7. Publish results
  8. Retest

Types of Evidence[edit]

There are many types of evidence gathering tools used in medicine. They all use the scientific method and are subject to rigorous scrutiny if they report a hypothesis as true.

Systematic Review[edit]

A systematic review is a literature review focused on a single question which tries to identify, appraise, select and synthesize all high quality research evidence relevant to that question. Systematic reviews are generally regarded as the highest level of medical evidence by evidence-based medicine professionals. An understanding of systematic reviews and how to implement them in practice is becoming mandatory for all professionals involved in the delivery of health care.

A systematic review is a summary of research (often in the biomedical or health care context) that uses explicit methods to perform a thorough literature search and critical appraisal of individual studies to identify the valid and applicable evidence. It often, but not always, uses appropriate techniques (meta-analysis) to combine these valid studies, or at least uses grading of the levels of evidence depending on the methodology used. A systematic review uses an objective and transparent approach for research synthesis, with the aim of minimizing bias. While many systematic reviews are based on an explicit quantitative meta-analysis of available data, there are also qualitative reviews which nonetheless adhere to the standards for gathering, analyzing and reporting evidence. Recent developments include realist reviews and the meta-narrative approach.[7]

Many healthcare journals now publish systematic reviews, but the best-known source is the Cochrane Collaboration, a group of over 6,000 specialists in health care who systematically review randomised trials of the effects of treatments and, when appropriate, the results of other research. Cochrane reviews are published in the Cochrane Database of Systematic Reviews section of the Cochrane Library, which to date (February 2007) contains 2,893 complete reviews and 1,646 protocols.


While systematic reviews are regarded as the strongest form of medical evidence, a review of 300 studies found that not all systematic reviews were equally reliable, and that their reporting could be improved by a universally agreed upon set of standards and guidelines.[8]

A further study by the same group found that of 100 guidelines reviewed, 4% required updating within a year, and 11% after 2 years; this figure was higher in rapidly-changing fields of medicine, especially cardiovascular medicine.[9] 7% of systematic reviews needed updating at the time of publication. [9]

Randomized Controlled Trials[edit]

A randomized controlled trial (RCT) is a type of scientific experiment most commonly used in testing healthcare services (such as medicine or nursing) or health technologies (such as pharmaceuticals, medical devices or surgery). According to Lachin (1998), 'RCTs are considered the most reliable form of scientific evidence in healthcare because they eliminate spurious causality and bias'. RCTs are mainly used in clinical studies, but are also employed in other sectors such as judicial, educational, and social research. As their name suggests, RCTs involve the random allocation of different interventions (or treatments) to subjects. This ensures that known and unknown confounding factors are evenly distributed between treatment groups.

In an open trial, the researcher knows the full details of the treatment, and so does the patient. These trials are open to challenge for bias, and they do nothing to reduce the placebo effect. However, sometimes they are unavoidable, particularly in relation to surgical techniques, where it may not be possible or ethical to hide from the patient which treatment he or she received. Usually this kind of study design is used in bioequivalence studies.

In a single-blind trial, the researcher knows the details of the treatment but the patient does not. Because the patient does not know which treatment is being administered (the new treatment or another treatment) there might be no placebo effect. In practice, since the researcher knows, it is possible for them to treat the patient differently or to subconsciously hint to the patient important treatment-related details, thus influencing the outcome of the study.

In a double-blind trial, one researcher allocates a series of numbers to 'new treatment' or 'old treatment'. The second researcher is told the numbers, but not what they have been allocated to. Since the second researcher does not know, they cannot possibly tell the patient, directly or otherwise, and cannot give in to patient pressure to give them the new treatment. In this system, there is also often a more realistic distribution of sexes and ages of patients. Therefore double-blind (or randomized) trials are preferred, as they tend to give the most accurate results.

Traditionally the control in randomized controlled trials refers to studying a group of treated patients not in isolation but in comparison to other groups of patients, the control groups, who by not receiving the treatment under study give investigators important clues to the effectiveness of the treatment, its side effects, and the parameters that modify these effects.

There are two processes involved in randomizing patients to different interventions. First is choosing a randomization procedure to generate a random and unpredictable sequence of allocations. This may be a simple random assignment of patients to any of the groups at equal probabilities, or may be complex and adaptive. A second and more practical issue is allocation concealment, which refers to the stringent precautions taken to ensure that the group assignment of patients are not revealed to the study investigators prior to definitively allocating them to their respective groups.

A major difficulty in dealing with trial results comes from commercial, political and/or academic pressure. Most trials are expensive to run, and will be the result of significant previous research, which is itself not cheap. There may be a political issue at stake (compare MMR vaccine) or vested interests (compare homeopathy). In such cases there is great pressure to interpret results in a way which suits the viewer, and great care must be taken by researchers to maintain emphasis on clinical facts.[10]

Cohort Study[edit]

A cohort study or panel study is a form of longitudinal study used in medicine and social science. It is one type of study design and should be compared with a cross-sectional study.

A cohort is a group of people who share a common characteristic or experience within a defined period (e.g., are born, leave school, lose their job, are exposed to a drug or a vaccine, etc.). Thus a group of people who were born on a day or in a particular period, say 1948, form a birth cohort. The comparison group may be the general population from which the cohort is drawn, or it may be another cohort of persons thought to have had little or no exposure to the substance under investigation, but otherwise similar. Alternatively, subgroups within the cohort may be compared with each other.

In medicine, a cohort study is often undertaken to obtain evidence to try to refute the existence of a suspected association between cause and disease; failure to refute a hypothesis strengthens confidence in it. Crucially, the cohort is identified before the appearance of the disease under investigation. The study groups, so defined, are observed over a period of time to determine the frequency of new incidence of the studied disease among them. The cohort cannot therefore be defined as a group of people who already have the disease. Distinguishing causality from mere correlation cannot usually be done with results of a cohort study alone.

The advantage of cohort study data is the longitudinal observation of the individual through time, and the collection of data at regular intervals, so recall error is reduced. However, cohort studies are expensive to conduct, are sensitive to attrition and take a long time to generate useful data.

Some cohort studies track groups of children from their birth, and record a wide range of information (exposures) about them. The value of a cohort study depends on the researchers' capacity to stay in touch with all members of the cohort. Some of these studies have continued for decades.[11]

Case-control Study[edit]

Case-control studies are used to identify factors that may contribute to a medical condition by comparing a group of patients who have that condition with a group of patients who do not.

Case-control studies are a relatively inexpensive and frequently-used type of epidemiological study that can be carried out by small teams or individual researchers in single facilities in a way that more structured trials often cannot be. They have pointed the way to a number of important discoveries and advances, but their retrospective, non-randomized nature limits the conclusions that can be drawn from them. Case-control studies use patients who already have a disease or other condition and look back to see if there are characteristics of these patients that differ from those who don’t have the disease.

The case-control study provides a cheaper and quicker study of risk factors; if the evidence found is convincing enough, then resources can be allocated to more "credible" and comprehensive studies. One major disadvantage of case-control studies is that they do not give any indication of the absolute risk of the factor in question. For instance, a case-control study may tell you that a certain behavior may be associated with a tenfold increased risk of death as compared with the control group. Although this sounds alarming, it would not tell you that the actual risk of death would change from one in ten million to one in one million, which is quite a bit less alarming. For that information, data from outside the case-control study must be consulted.[12]

Outcomes Research[edit]

Outcomes research is the method by which the effectiveness of a health care system, unit or individual treatment is measured. The measurements used may be objective measures, such as blood pressure readings, or subjective measures, such as the severity of pain. It allows for a somewhat objective measure of the outcomes of clinical practice.[13]

Ecological Studies[edit]

Ecological studies are studies in which associations are sought between a disease and certain ecological attributes of the population. Ecological studies may reveal an increased incidence of disease in certain areas of a country, at a certain time of year, among migrant workers, among people of a certain occupation or people of a certain social class.[14]

Case-series[edit]

A case series (also known as a clinical series) is a medical research study that tracks patients with a known exposure or examines their medical records for exposure and outcome. A case series can be retrospective or prospective and usually involves a smaller number of patients than more powerful case-control studies or randomized controlled trials.

A case series is a type of observational study. Case series may be consecutive or non-consecutive, depending on whether all cases presenting to the reporting authors over a period of time were included, or only a selection.[15]

Expert Opinion[edit]

This is the knowledge available to experts in their field based on experience and training. It is considered the least valuable source of medical evidence and the experts are required to update and improve their knowledge by studying medical evidence higher up the valued chain.[16]

Ranking of Evidence[edit]

Medical evidence is ranked according to its value:[17]

  • 1a: Systematic reviews (with homogeneity ) of randomized controlled trials
  • 1a-: Systematic review of randomized trials displaying worrisome heterogeneity
  • 1b: Individual randomized controlled trials (with narrow confidence interval)
  • 1b-: Individual randomized controlled trials (with a wide confidence interval)
  • 1c: All or none randomized controlled trials
  • 2a: Systematic reviews (with homogeneity) of cohort studies
  • 2a-: Systematic reviews of cohort studies displaying worrisome heterogeneity
  • 2b: Individual cohort study or low quality randomized controlled trials (<80% follow-up)
  • 2b-: Individual cohort study or low quality randomized controlled trials (<80% follow-up / wide confidence interval)
  • 2c: 'Outcomes' Research; ecological studies
  • 3a: Systematic review (with homogeneity) of case-control studies
  • 3a-: Systematic review of case-control studies with worrisome heterogeneity
  • 3b: Individual case-control study
  • 4: Case-series (and poor quality cohort and case-control studies)
  • 5: Expert opinion without explicit critical appraisal, or based on physiology, bench research or 'first principles'


Lesson Summary

The ranking of medical evidence is as follows:

  • Systematic Review
  • Randomised Controlled Trial
  • Cohort Study
  • Case-control study
  • Outcomes research
  • Ecological study
  • Case-series
  • Expert opinion
Any Questions?

How to rank Cross-sectional studies? Please guide about that too.

References[edit]

  1. Sackett DL, Rosenberg WM, Gray JA, Haynes RB, Richardson WS (1996). "Evidence based medicine: what it is and what it isn't". BMJ 312 (7023): 71–2. PMID 8555924. http://www.bmj.com/cgi/content/full/312/7023/71. 
  2. Elstein AS (2004). "On the origins and development of evidence-based medicine and medical decision making". Inflamm. Res. 53 Suppl 2: S184–9. doi:10.1007/s00011-004-0357-2. PMID 15338074. 
  3. G. Ellrodt, D. J. Cook, J. Lee, M. Cho, D. Hunt and S. Weingarten. Evidence-based disease management. JAMA Vol. 278 No. 20, November 26, 1997.
  4. Tonelli, MR (2001). "The limits of evidence-based medicine". Respiratory Care 46 (12): 1435–1440. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11728302&dopt=Abstract. 
  5. David L Sackett, William M C Rosenberg, J A Muir Gray, R Brian Haynes, W Scott Richardson. Evidence based medicine: what it is and what it isn't. BMJ 1996;312:71-72
  6. How to think like a scientist: Answering questions by the scientific method. Stephen P Kramer
  7. Diffusion of Innovations in Health Service Organisations: a systematic literature review
  8. Moher D, Tetzlaff J, Tricco AC, Sampson M, Altman DG (2007). "Epidemiology and reporting characteristics of systematic reviews". PLoS Med. 4 (3): e78. doi:10.1371/journal.pmed.0040078. PMID 17388659. 
  9. 9.0 9.1 Shojania KG, Sampson M, Ansari MT, Ji J, Doucette S, Moher D (2007). "How quickly do systematic reviews go out of date? A survival analysis". Ann. Intern. Med. 147 (4): 224–33. PMID 17638714. 
  10. Lachin JM, Matts JP, Wei LJ (Dec 1988). "Randomization in Clinical Trials: Conclusions and Recommendations". Controlled Clinical Trials 9 (4): 365-74. PMID 3203526
  11. Statistical Methods in Cancer Research: Volume II: The Design and Analysis of Cohort Studies (Iarc Scientific Publication, No 82). N. E. Breslow and N. E. Day.
  12. Case Study Research: Design and Methods, Third Edition, Applied Social Research Methods Series, Vol 5. Robert K. Yin
  13. Understanding Health Care Outcomes Research. Robert L. Kane
  14. Analysing Ecological Data (Statistics for Biology and Health). Alain F. Zuur, Elena N. Ieno, and Graham M. Smith
  15. Brain Imaging: Case Review Series. Laurie A. Loevner
  16. Epidemiology: Beyond the Basics. M. Szklo and F. Javier Nieto
  17. Oxford Centre for Evidence-based Medicine Levels of Evidence and Grades of Recommendation