STUDY DESIGNS WHICH ONE IS BEST?

Transcription

1 STUDY DESIGNS WHICH ONE IS BEST? David Nunan, PhD Research Fellow and Tutor Nuffield Department of Primary Care Health Sciences and Oxford Centre for Evidence Based Medicine University of Oxford

2 Exercise What is the best study design? Why? Compare with your neighbour

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4 Does thalidomide cause phocomelia?

5 You should be happy if when you leave today, you understand Observational studies Descriptive studies Case studies, case series Analytic studies Cohort, case-control, cross-sectional Experimental studies (randomised trials) Qualitative studies

6 THE LANCET 2002;359:57-61 Basic principles of study design

7 Bias in study design Least biased Randomised controlled trials Experimental Cohort studies Case-control studies Observational Most biased Cross-sectional studies Clinical observation (case reports, case-series)

8 Practicing EBM the 4 A s Clinical questions: Interventions Aetiology and risk factors Diagnosis Prognosis Frequency and rate Step 1 Ask a clinical question Step 2 Acquire the best evidence Step 3 Appraise the evidence Step 4 Apply the evidence

9 What study designs should you be looking for? Question Question type Best study design What should I do about this condition or problem? What causes the problem? Does this person have the condition or problem? What will happen with this problem? How common is the problem? INTERVENTION AETIOLOGY AND RISK FACTORS DIAGNOSIS PROGNOSIS AND PREDICTION FREQUENCY AND RATE RANDOMISED CONTROLLED TRIAL (RCT) > cohort RCT (?) > cohort > case control CROSS-SECTIONAL (random or consecutive sample) COHORT (survival) > case control > case series CROSS SECTIONAL (prevalence) COHORT (incidence)

10 CASE REPORT/SERIES

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12 Case reports

13 Conclusions case reports/series Advantages Can help identify new trends or diseases Can have significant influence on subsequent literature and possibly on clinical practice Often, report rare conditions for which trials may not be feasible. Disadvantages Generally short term Investigator selection bias Generally no controls Design pitfalls to look out for Patient(s) should be described in detail Carefully reported, unbiased observations Explore and infer, not confirm, deduce or prove

14 Overview Observational studies Descriptive studies Case studies, case series Analytic studies Cohort, case-control, cross-sectional Experimental studies (randomized trials) Qualitative studies

15 Analytical Studies

16 What is the study design?

17 Analytical Studies

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19 Lowest = furthest from base station Highest = nearest to base station Is there a better study design to answer the question Mobile phone masts and childhood cancer?

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21 Conclusions case control studies Advantages Good for studying rare conditions or diseases Useful as initial studies to establish association Can answer questions that can t be answered by other designs Disadvantages Retrospective studies have more problems with data quality (recall bias) Limited to examining one outcome Not good for study of rare exposures It can be difficult to find a suitable control group Design pitfalls to look out for Confounding exposure and outcome related to third variable

22 What is the study design?

23 Analytical Studies

24 Bias in cross-sectional studies Risk of ASthma in Poultry breeders The RASP study a cross sectional survey How might risk be underestimated?

25 # of peptic ulcer cases (weekly) Bias in cross-sectional studies Peptic Ulcers and milk ingestion The PUKE study a cross sectional study R² = 0,9713 Does milk cause peptic ulcers? Milk consumption (litres/day)

26 Conclusions cross sectional studies Advantages Good for assessing prevalence/burden of disease Quick and easy to conduct (no follow-up) Multiple outcomes and exposures can be studied Useful to identify hypothesis/relationships that can be followed up in experimental studies Disadvantages Cannot measure change in variables over time Inappropriate for demonstrating causal relationships Not suitable for rare diseases or disease of short duration Non-responder bias (surveys)

27 Analytical Studies

28 Cohort studies Prospective or retrospective Controlled Can determine causes and incidence of diseases as well as identify risk factors Generally expensive and difficult to carry out (prospective) Cheaper and quicker = retrospective

29 Cohort Design - Prospective Study population free of disease Exposure present Exposure absent present Disease/outcome No disease/outcome Disease/outcome No disease/outcome future Time Study begins here

30 Cohort Design - Retrospective Study population free of disease Exposure present Exposure absent present Disease/outcome No disease/outcome Disease/outcome No disease/outcome future Time Study begins here

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33 2748 men and women, years HDL-C measured at baseline Followed for 12 years Death from all causes, CHD, and CVD No loss to follow-up

34 Low levels of HDL-C associated with increase mortality Risk of death from all-causes 2x higher in low HDL-C versus high HDL-C Risk of death from CVD and CHD 4x higher in low versus high HDL-C

35 Conclusions cohort studies Advantages Subjects in cohorts can be matched, which limits the influence of confounding Standardisation of criteria/outcome is possible Easier and cheaper than a RCT Disadvantages Cohorts can be difficult to identify due to confounding variables No randomisation, which means that imbalances in patient characteristics could exist Blinding/masking difficult Can take long time for outcome of interest to occur

36 Confounding Other patient features/causal factors, apart from the one being measured, that can affect the outcome of the study e.g..

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38 Why random allocation? Non-randomised controlled trials: Can detect associations between an intervention and an outcome But, they cannot rule out the possibility that the association was caused by a third factor linked to both intervention and outcome. Failure to conceal random allocation exaggerates estimates of treatment effects Odds ratios were exaggerated by 41% for inadequately concealed trials and by 30% for unclearly concealed trials (adjusted for other aspects of quality).

39 RANDOMISATION Study population free of disease Exposure present Exposure absent present Disease/outcome No disease/outcome Disease/outcome No disease/outcome future Time Study begins here

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42 Difference between randomized trials and nonrandomized studies Randomized trials Random (and hopefully concealed) procedure decides which participants go in which arm. (Rules out allocation bias) Placebo controls and double blinding possible. Non-randomized/Cohort studies Participants self-select or are selected to receive an intervention/exposure Placebo controls and double blinding not possible.

43 What study design(s) would you choose? 1. What is the importance of patient preferences in the choice of treatment for benign prostatic hyperplasia? (1 min) 2. How will you identify and describe misunderstandings between patients and doctors associated with prescribing decisions in general practice? (1 min) 3. How can you explore how men and women with cancer talk about using the internet? (1 min)

44 Overview Observational studies Descriptive studies Case studies, case series Analytic studies Cohort, case-control, cross-sectional Experimental studies (randomized trials) Qualitative studies

45 What is qualitative research? Study of things (phenomena) in their natural settings Make sense of, or interpret, phenomena in terms of meanings people bring to them Aim to get a deeper significance of what study participants ascribe to the research topic Interpretive, naturalistic approach Priority is how participant(s) experiences contribute to the research question

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47 What are some qualitative research methods? Participant observation data on naturally occurring behaviours in their usual contexts Focus groups effective in eliciting data on cultural norms of a group and generating broad overviews of issues in groups In-depth interviews optimal for collecting data on individuals personal histories, perspectives, and experiences

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50 You should be happy if when you leave today, you understand Observational studies Descriptive studies Case studies, case series Analytic studies Cohort, case-control, cross-sectional Experimental studies (randomised trials) Qualitative studies

51 THANK YOU FOR LISTENING! THANK YOU FOR LISTENING!