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Study design and Levels of Evidence

Study design and Levels of Evidence. Dr Rafael Perera Director of Research Methods. Research questions. determine problem ( case reports/case-series , ecological/cross-sectional studies ) design interventions

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Study design and Levels of Evidence

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  1. Study design and Levels of Evidence Dr Rafael Perera Director of Research Methods

  2. Research questions determine problem (case reports/case-series, ecological/cross-sectional studies) design interventions (randomized controlled trials) find potential causes (case-control/cohort studies) implement activities (surveillance studies, meta-analyses)

  3. Taxonomy of trial design

  4. Hierarchy of Evidence

  5. Levels of Evidence

  6. Case Report Series Describe patients’ characteristics, and may generate ideas for future studies

  7. Randomized Controlled Trial Population Meet Inclusion Criteria? Sample Baseline Assessment Follow-up assessments • Typical RCT randomises two (or more) groups of patients to different treatments

  8. Observational Studies • Ecological • Cross-sectional • Case-control • Cohort or “follow-up” studies

  9. Ecological study • Focuses on the characteristics of population groups rather than their individual members. • The group could be defined by • time (calendar period, birth cohort), • geography (country, city), • socio-demographic characteristics (ethnicity, religion). • Used to examine the differential distribution of diseases among people with different risk profiles. • The kinds of comparisons usually take advantage of routinely collected data and are therefore inexpensive

  10. Cross-sectional study Population Group of interest (e.g. smokers) Compute prevalence of stroke in smokers sample Compare groups Comparison Group of interest (e.g. non-smokers) Compute prevalence of stroke in non-smokers Starting point Present • A cross-sectional study is a single “snapshot” in time • We can only study current risk factors and diseases (prevalence)

  11. Case-control study Population of diseased individuals Sample of diseased individuals Sample of non-diseased individuals Population of non-diseased individuals Exposure History Past Starting point • Case-control studies examine the association of disease with past exposure (s)

  12. Cohort study Population of disease free individuals Sample Starting point Future • Selected group of disease-free people who are classified according to a specific exposure. • Observed over time to see who develops the disease or outcomes (s) of interest. • Can measure incidence (new cases of disease) and thus risk

  13. Principles of Evidence Grading 1. Different research evidence for different types of clinical questions 2. Top level of evidence – Systematic Reviews (Irrespective of type of clinical question) 3. When SR not available, efficient strategies required to identifying relevant evidence 4. Study “Level” alone should not be used to grade evidence 5. Balanced assessments use different types of research

  14. Bias in RCTs Effect of study features on effect size in 229 trials

  15. Bias in Diagnostic Studies Lijmer JG et al. JAMA 1999;282:1062-1067

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