Public Health

Study Designs and Hierarchy of Evidence

• Filtered articles are the fastest way to access evidence
• Unfiltered articles are the building blocks for any systematic review, meta-anaysis, practice guideline, or other article synopses

This figure shows the tree of possible designs, branching into subgroups of study designs by whether the studies are descriptive or analytic and whether the analytic studies are experimental or observational. The list is not completely exhaustive but covers most basic designs.

Check this PDF about study designs and their definitions.

Source: Centre for Evidence-Based Medicine, Study Designs

Glossary of EBM terms

Check the following dictionaries and glossaries for definitions for each of these types of studies:

• Centre for Evidence-Based Medicine
• Evidence Based Emergency Medicine

Levels of Evidence

• GRADE Handbook

  The Grading of Recommendations Assessment, Development and Evaluation (short GRADE) Working Group began in the year 2000 as an informal collaboration of people with an interest in addressing the shortcomings of present grading systems in health care. The working group has developed a common, sensible and transparent approach to grading quality of evidence and strength of recommendations. Many international organizations have provided input into the development of the approach and have started using it.

• Levels of Evidence and Grades of Recommendations

  Centre for Evidence Based Medicine

• Ebell, M. H., Siwek, J., Weiss, B. D., Woolf, S. H., Susman, J., Ewigman, B., & Bowman, M. (2004). Strength of Recommendation Taxonomy (SORT): A Patient-Centered Approach to Grading Evidence in the Medical Literature. The Journal of the American Board of Family Practice, 17(1), 59–67. https://doi.org/10.3122/jabfm.17.1.59

Focusing Clinical Questions

A clinical question needs to be directly relevant to the patient or problem at hand and phrased in such a way as to facilitate the search for an answer. PICO makes this process easier. It is a mnemonic for the important parts of a well-built clinical question. It also helps formulate the search strategy by identifying the key concepts that need to be in the article that can answer the question.

PICO or PICOT or PICOTT

PATIENT OR PROBLEM How would you describe a group of patients similar to yours? What are the most important characteristics of the patient? INTERVENTION, EXPOSURE, PROGNOSTIC FACTOR What main intervention, exposure, or prognostic factor are you considering? What do you want to do with this patient? COMPARISON What is the main alternative being considered, if any? OUTCOME What are you trying to accomplish, measure, improve or affect? Type of Question Therapy / Diagnosis / Harm / Prognosis / Prevention Type of Study Systematic review / RCT / cohort study / case-control

Sources: 

• Richardson, W. S., Wilson, M. C., Nishikawa, J., & Hayward, R. S. (1995). The well-built clinical question: A key to evidence-based decisions. ACP Journal Club, 123(3), A12-13.
• Nursingessaywriting.com team. (2022). What Is PICOT? - Evidence-Based Medical and Nursing Practice [Blog]

Types of Questions

Primary Question Types

• Therapy: how to select treatments to offer our patients that do more good than harm and that are worth the efforts and costs of using them.
• Diagnostic tests: how to select and interpret diagnostic tests, in order to confirm or exclude a diagnosis, based on considering their precision, accuracy, acceptability, expense, safety, etc.
• Prognosis: how to estimate a patient's likely clinical course over time due to factors other than interventions
• Harm / Etiology: how to identify causes for disease (including its iatrogenic forms).

Other Question Types

• Clinical findings: how to properly gather and interpret findings from the history and physical examination.
• Clinical manifestations of disease: knowing how often and when a disease causes its clinical manifestations and how to use this knowledge in classifying our patients' illnesses.
• Differential diagnosis: when considering the possible causes of our patient’s clinical problem, how to select those that are likely, serious and responsive to treatment.
• Prevention: how to reduce the chance of disease by identifying and modifying risk factors and how to diagnose disease early by screening.
• Qualitative: how to empathize with our patients’ situations, appreciate the meaning they find in the experience and >understand how this meaning influences their healing.

From: Sackett, D.L. Evidence-based medicine: how to practice and teach EBM.

Clinical Question AND Study Design

The type of question will often dictate the best study design to address the question. In the absence of the best study design, move down the hierarchy of evidence:

*it is not always ethical to randomize people to a known harmful exposure. However, some RCTs do contain information on adverse events, side effects, etc. that could be helpful in answering certain clinical questions regarding harms. 

Creating a Search

STEP 1: Using PICO to formulate a search question

STEP 2: Turn search terms into a search strategy.

STEP 3: Turn your search strategy into results.

This is a detailed demonstration of using many of the basic and advanced features of PubMed.

(1) TRIP (Turning Research Into Practice) Database Plus

Simultaneously searches evidence-based sources of systematic reviews, practice guidelines, and critically appraised topics and articles. Also searches MEDLINE’s Clinical Queries, medical image databases, e-textbooks, and patient information leaflets.

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(2) Filtered resources appraise the quality of studies and often make recommendations for practice:

• Systematic Reviews are searchable in the following sources: ​
  • CINAHL (Cumulative Index to Nursing and Allied Health Literature) - Enter your search query and add Systematic Reviews in a separate box. Or check the box Evidence-Based Practice in Advanced Search screen.  
  • Database of Abstracts of Reviews of Effects (DARE)
  • MEDLINE - Search limiters include Evidence-Based Medicine Reviews (EBM Reviews), various types of Clinical Queries, and Publication Types in Advanced Search screen. You can choose to limit results to clinical trials, systematic reviews, case reports, comparative studies or meta-analysis.
  • PubMed Clinical Queries - Enter your search query. Results will be displayed under the 2nd column, Systematic Reviews. 
  • PEDro (Physiotherapy Evidence Database) is a free database of over 29,000 randomized trials, systematic reviews and clinical practice guidelines in physiotherapy.

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• Critically-Appraised Individual Articles. Authors of critically-appraised individual articles evaluate and synopsize individual research studies.
  • The ACP Journal Club. The editors of this journal screen the top 100+ clinical journals and identify studies that are methodologically sound and clinically relevant. An enhanced abstract, with conclusions clearly stated, and a commentary are provided for each selected article. Published by the American College of Physicians-American Society of Internal Medicine.
  • BMJ Updates. Quality articles from over 110 clinical journals are selected by research staff, and then rated for clinical relevance and interest by an international group of physicians. Includes a searchable database of the best evidence from the medical literature and an email alerting system. From BMJ Publishing Group and McMaster University's Health Information Research Unit.

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(3) Unfiltered Resources. Evidence is not always available via filtered resources. Searching the primary literature may be required.

• PubMed Clinical Queries. To limit your PubMed search to the best evidence-producing studies:

1. Enter your search term(s).
2. Results will display under Clinical Study Categories, Systematic Reviews, Medical Genetics.
3. For Clinical Study Categories you can filter by "category" (etiology, diagnosis, therapy, prognosis, clinical prediction guidelines) or "scope" (broad or narrow)
4. For Medical Genetics you can filter by "topic" (diagnosis, differential diagnosis, clinical description, clinical description, management, genetic counseling, molecular genetics, genetic testing)

• PsycINFO - International coverage of the professional and academic literature in psychology, medicine, psychiatry, nursing, sociology, education, pharmacology, physiology, linguistics, and other areas. Click Browse Methodology and select the checkbox for Systematic Reviews.
• CINAHL (Cumulative Index to Nursing and Allied Health Literature). To limit your CINAHL search to the best evidence-producing studies: select type of Clinical Queries or select Evidence-Based Care Sheet or Practice Guidelines or Systematic Reviews in “Publication Type”.
• ClinicalTrials.gov is a registry of federally and privately supported clinical trials conducted in the United States and worldwide. It gives you information about a trial's purpose, who may participate, locations, and phone numbers for more details. This information should be used in conjunction with advice from healthcare professionals.

Critical Appraisal Worksheets

• Clinical Decision Analysis
• Diagnosis
• Differential Diagnosis
• Economic Analysis
• Harm
• Practice Guidelines
• Prognosis
• Qualitative Studies
• Quality Improvement
• Screening
• Systematic Reviews
• Therapy

This list is adapted from the Duke University Medical Center Library & Archives, Evidence-Based Practice LibGuide, licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Training Resources

Visit resources from Duke University for further information on EBP teaching and professional development workshops: Teaching EBM Professional Development Workshops BMJ EBM Toolkit - The BMJ's free EBM Toolkit enables individuals to learn, practice and discuss EBM, and provides EBM tools. There is information introducing the key methods of EBM, such as clarifying a clinical question, designing a search, and appraising, synthesizing, and assessing the quality of the evidence. Additionally, there is information intended to support and promote discussion around EBM.