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Choosing your study design

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Choosing your study design

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Choosing your study design

  1. 1. Choosing the appropriate study design Research Week 2014 Health Cluster UKM
  2. 2. USUAL STUDY DESIGNS OBSERVATIONAL STUDY EXPERIMENTAL STUDY DESCRIPTIVE STUDY ANALYTIC STUDY CROSS-SECTIONAL CASE CONTROL COHORT CLINICAL TRIAL COMMUNITY TRIAL
  3. 3. Difference Between Study Designs Study Design Intervention Temporal Sequence Sampling Cross Sectional Absent Risk Factor and Outcome at same time. Yes Case Control Absent Outcome first, then risk factor (retrospectively). Matching Cohort Absent Risk Factor first, then Outcome. Maybe Clinical/ Community Trial Present Intervene, then measure Outcome. Randomisation
  4. 4. Which design is appropriate for your study?
  5. 5. Usual study design Commonly chosen in PPUKM • Clinical trial • Cross-sectional • Case-control • Cohort (PURE & Malaysian Cohort) Specific for Patho/Diagnostic • Diagnostic testing • Sensitivity, Specificity & ROC • Kappa/ Agreement
  6. 6. Which one? • If you have an intervention, then it is a clinical trial. • If there is no intervention, then it is one of the observational studies, usually cross-sectional. • If the disease is rare, less than 5% prevalence rate, then it is a case-control study. • If you are looking forward in time for new incidence of the disease, then it is a cohort study.
  7. 7. Clinical Trials If you have an intervention, then probably it is a clinical trial.
  8. 8. Clinical Trial Time Intervention Confounders Outcome Look forward in time Starts with Intervention, then measure the future Outcome
  9. 9. EXPERIMENTAL STUDY Eligible Participants T + T - C + C + C - C - Future Randomisation Study Population Selection
  10. 10. Clinical Trial Drug Sogood Drug Feelgood No improvement (25%) Improved (75%) Improved (70%) No improvement (30%) Sample ratio (1:1)Depressed patients Intervention Outcome-Improved? Starts with Intervention, then measure the future Outcome Time Future
  11. 11. PROCEDURE • Assignment of subjects to treatments • Interview and examinations • Methods of assessment • Lab. Studies • Treatment schedules: number of units per visit, rules for changing dosage, compliance checks • Adverse reactions: definition and grading, inquiry, management. • Drops out: definition, handling and recording, terminating and extending study
  12. 12. CROSS-SECTIONAL STUDY Also known as Prevalence Study or Survey. Easiest study design.
  13. 13. Cross-Sectional Study • Measures the relationship of variables in a defined population at one particular time • Both risk factors (exposure) and disease outcome are observed at the same (point in) time in a sample (or the entire population) of subjects. • i.e. Studying the effect of overweight on the prevalence rate of diabetes mellitus.
  14. 14. Exposure & Outcome Time Risk Factor Confounders Outcome
  15. 15. Cross-Sectional Study Time Risk Factor Confounders Outcome Both Risk Factor & Outcome measured at the same time.
  16. 16. Cross-Sectional Study Overweight (40%) Normal (60%) Disease - (68%) Disease + (32%) Disease + (8%) Disease - (92%) Sample ratio Risk Factor -Overweight Outcome-Diabetes Mellitus Both Risk Factor & Outcome measured at the same time. Time
  17. 17. Reminder • If the prevalence/incidence rate is below 5%, please do not choose a cross-sectional study design. Your number of cases would be too low to do any analysis. • For example, doing a cross-sectional study of HIV cases among Malaysian pregnant mothers. The rate of HIV+ is only 0.04%. Therefore even with 10,000 samples, you will only get 4 HIV+. (http://www.moh.gov.my/attachments/3886.pdf)
  18. 18. Imagine a result like this HIV+ HIV- Total Risk Factor + 3 97 100 Risk Factor - 1 9899 9900 4 9996 10000 Not much analysis could be done with a result like this. So instead do a case-control study for rare diseases.
  19. 19. CASE-CONTROL STUDY For rare diseases. Consider if prevalence rate below 5%.
  20. 20. CASE CONTROL STUDY - CONCEPT • comparison of group of diseased person (cases) with another group of non-diseased person (control) for past exposure to a suspected cause of the disease. • arises because of hypothesis that the risk factor (exposure) causes the disease
  21. 21. Case-Control Study Time Exposure Confounders Outcome Look back in time Starts with Outcome, then trace the retrospective exposure
  22. 22. Case-Control Study Cataract Normal vision DM - (50%) DM + (50%) DM + (8%) DM - (92%) Sample ratio (1:1) Outcome-Cataract Risk Factor-Diabetes Mellitus Starts with Outcome, then trace the retrospective exposure Time Past
  23. 23. CASE SELECTION • Determine clear and reproducible definitions of the health problems to be studied (avoid misclassification bias) • source of cases All persons with the disease seen in particular facility(ies) in a specified period of time. All persons with the disease found in general population. • Incidence cases (newly diagnosed cases) preferred
  24. 24. CHOICE OF CONTROLS • Controls should ideally be selected from the same population gave rise to cases • Similar to cases in regard to past potential exposure • Free from study disease • If controls are patient with other diseases then select only diseases that are not known to have relationship with factors under study.
  25. 25. ADVANTAGES • able to study rare diseases • can explore multiple exposures • relatively inexpensive • can calculate Odds Ratio • can support causation but not prove it • easy to get cases
  26. 26. COHORT STUDY - ignore, none of you will do this.
  27. 27. COHORT STUDY BASIC CONCEPT • Group or groups of individuals are studied over time as to onset of new cases of disease and factors associated with the onset of disease. • Synonyms : incidence study, longitudinal study, prospective study.
  28. 28. Cohort Study Time Exposure Confounders Outcome Look forward in time Starts with Risk Factor, then measure the future Outcome
  29. 29. COHORT STUDY DISEASE Free from disease EXPOSED GROUP UNEXPOSED GROUP DISEASE NONDISEASE NONDISEASE FOLLOW-UP
  30. 30. Cohort Study Normal Overweight DM - (68%) DM + (32%) DM + (7%) DM - (93%) Sample ratio (1:1) Risk Factor-Weight Outcome-Diabetes Mellitus Starts with Risk Factor, then measure the future Outcome Time Future Free from DM
  31. 31. Recruitment • Those recruited must be free from the disease of interest at the beginning of the study. • Those with sub-clinical presentations of the disease may miss from being excluded. This is one of the challenges.
  32. 32. Follow-up • Keep participation at > 90% • Must have equal ability to detect disease in all subjects and all groups, with standard measurement • Active vs Passive follow-up • Verbal Autopsy • Blinding of the assessor • Assess both primary and secondary outcomes
  33. 33. Next Calculate sample size
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Choosing your study design

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