Download - final study design (2)
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HANAN ABBAS ASSISTANT PROFESSOR
OF FAMILY MEDICINE
STUDY DESIGN
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Study Design
Objectives By the end of this session candidates should be
able to: 1. Review the fundamental study designs 2. Discuss Studies -Uses -Strengths/weaknesses
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Research Design
• Most problems in studies are due to poor design (not poor analysis)
• The research design is the master plan specifying the methods and procedures for collecting and analyzing the needed information.
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Study Design
Experimental
• Randomized
controlled trials
• Quasi
Observational
Analytic • Cross sectional
• Case-control
• Cohort studies
Descriptive
• Case report
• Case series
• Survey
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Analytical Studies
Observational
Experimental
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Observational Studies
Cross-sectional
Case-control
Cohort
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Epidemiologic Study Designs
Grimes & Schulz, 2002 (www)
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Hierarchy of Epidemiologic Study Design
Tower & Spector, 2007 (www)
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Case reports
It describes the medical history or adverse
treatment effect for a patient .
advantages : generating ideas, save time.
Not expensive.
Disadvantages; weak evidence.
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Cross-sectional Study
Data collected at a single point in time
Describes associations
Prevalence
A “Snapshot”
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Example of a Cross-Sectional Study
Association between garlic consumption
and CAD in the Family Practice Clinic
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Cross-Sectional Study
Strengths
Quick
Cheap
Weaknesses
Cannot establish cause-effect
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Observational Studies
Case-Control Study
Start with people who have disease
Match them with controls that do not
Look back and assess exposures * case-control
(retrospective ): Begins with outcome (cancer cases and healthy controls
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CASE CONTROL STUDY
Risk factor absent
Risk factor present
Risk factor absent
Risk factor present
Taking into consideration the confounding variables e.g. age, sex, education, etc.
PRESENT PAST
Control
Case
Compare
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Example of a Case-Control Study
Are those with CAD less likely to have
consumed garlic?
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Advantages of Case-Control Studies
- Cheap, easy and quick studies
- Multiple exposures can be examined
- Rare diseases and diseases with long latency can be studied
- Suitable when randomization is unethical
(alcohol and pregnancy outcome)
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Disadvantages of Case-Control Studies
- Case and control selection troublesome
- Subject to bias (selection, recall, misclassification)
- Direct incidence estimation is not possible
- Temporal relationship is not clear
- Multiple outcomes cannot be studied
- If the incidence of exposure is high, it is difficult to show the difference between cases and controls
- Not easy to estimate attributable fraction
- Reverse causation is a problem in interpretation - especially in molecular epidemiology studies
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Case-Control Studies: Strengths
Good for rare outcomes: cancer
Can examine many exposures
Useful to generate hypothesis
Fast
Cheap
Provides Odds Ratio
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Case-Control Studies: Weaknesses
Cannot measure
Incidence
Prevalence
Relative Risk
Can only study one outcome
High susceptibility to bias
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Design pitfalls to look out for
Is the control group appropriate for the population?
Does the study use matching or pairing appropriately to avoid the effects of a confounding variable?
Does it use appropriate inclusion and exclusion criteria?
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Real-life Examples
Smedby, K. E., Hjalgrim, H., Askling, J., Chang, E. T., Gregersen, H., & Porwit-MacDonald, A. et al. (2006). Autoimmune and chronic inflammatory disorders and risk of non-hodgkin lymphoma by subtype. Journal of the National Cancer Institute, 98(1), 51-60.
This study matched patients with non-Hodgkin lymphoma (NHL) with control subjects and compared their history of autoimmune and chronic inflammatory disorders, markers of severity, and treatment. It found that the risk of NHL was increased in association with rheumatoid arthritis, primary Sjögren syndrome, systemic lupus erythematosus, and celiac disease.T
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Cohort Study
Begin with disease-free patients
Classify patients as exposed/unexposed
Record outcomes in both groups
Compare outcomes using relative risk
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Example of a Cohort Study
To see the effects of garlic use on CAD
mortality in a population
cohort (prospective): Begins with an exposure (smokers and non-smokers)
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Examples of Cohort Studies
* Framingham Heart Study (www)
* NHANES Studies (www)
* MACS (www)
* Physicians' Health Study (www)
* Nurses' Health Study (www)
* ALSPAC (www)
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Advantages of Cohort Studies
- Can establish population-based incidence
- Accurate relative risk (risk ratio) estimation
- Can examine rare exposures (asbestos > lung cancer)
- Temporal relationship can be inferred (prospective design)
- Time-to-event analysis is possible
- Can be used where randomization is not possible
- Magnitude of a risk factor’s effect can be quantified
- Selection and information biases are decreased
- Multiple outcomes can be studied
(smoking > lung cancer, COPD, larynx cancer)
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Disadvantages of Cohort Studies
- Lengthy and expensive
- May require very large samples
- Not suitable for rare diseases
- Not suitable for diseases with long-latency
- Unexpected environmental changes may influence the association
- Non-response, migration and loss-to-follow-up biases
- Sampling, ascertainment and observer biases are still possible
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Cohort Study: Strengths
Provides incidence data
Establishes time sequence for causality
Eliminates recall bias
Allows for accurate measurement of exposure
variables
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Cohort Study: Strengths
Can measure multiple outcomes
Can adjust for confounding variables
Can calculate relative risk
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Cohort Study: Weaknesses
Expensive
Time consuming
Cannot study rare outcomes
Confounding variables
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Cohort Study: Weaknesses
Exposure may change over time
Disease may have a long pre-clinical phase
Attrition of study population
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Design pitfalls to look out for
Design pitfalls to look out for
The cohorts need to be chosen from separate, but similar, populations.
How many differences are there between the control cohort and the experiment cohort?
Will those differences cloud the study outcomes?
Fictitious Example
A cohort study was designed to assess the impact of sun exposure on skin damage in beach volleyball players.
Comparisons of skin damage were then made based on the use of sunscreen.
The analysis showed a significant difference between the cohorts in terms of the skin damage.
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Real-life Example
Ramchand, R., Ialongo, N. S., & Chilcoat, H. D.
(2007). The effect of working for pay on adolescent tobacco use. American Journal of Public Health, 97(11), 2056-2062.
This study uses data collected from high school students from Baltimore, Maryland, and studies the differences in initiation of tobacco use between a cohort of adolescents that started working for pay and a cohort of adolescents that did not work. The results suggest that adolescents who work for pay have a higher risk of initiating tobacco use.
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Lindenauer, P. K., Rothberg, M. B., Pekow, P. S., Kenwood, C., Benjamin, E. M., & Auerbach, A. D. (2007). Outcomes of care by hospitalists, general internists, and family physicians. New England Journal of Medicine, 357(25), 2589-2600.
To study effects of hospitalists, general internists, and family physicians on patient care, patients that were hospitalized with certain conditions under the care of hospitalists, general internists, and family physicians were separated into three cohorts. The results showed that patients cared for by hospitalists had shorter hospital stays and lower costs than those cared for by general internists or family physicians.
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INTERVENTION STUDIES
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Experimental Studies
Clinical trials provide the “gold standard” of
determining the relationship between garlic
and cardiovascular disease prevention.
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Characteristics of the Experimental
Study a- Intervention
b- Control
c- Randomization to eliminate the confounding variables
d- blindness.
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Clinical Trials
Randomized
Double-blind
Placebo-controlled
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Clinical Trials
Strengths:
Best measure of causal relationship
Best design for controlling bias
Can measure multiple outcomes
Weaknesses:
High cost
Ethical issues may be a problem
Compliance
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time Study begins here (baseline point)
Study population
Intervention
Control
outcome
no outcome
outcome
no outcome
baseline
future
RANDOMIZATION
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Types of trials
Blinded Not blinded
Randomised Not randomised
Controlled Not controlled
Trial
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Double-blind design • Neither patient nor outcome evaluator knows Rx to which patient was assigned • Single-blind • Patient or evaluator is blinded as to Rx, but not both • Triple-blind • Patient, Physician, and Data analyst are blinded as to Rx identity
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Design pitfalls to look out for
An RCT should be a study of one population only.
Was the randomization actually “random,” or are there really two populations being studied?
The variables being studied should be the only variables between the experimental group and the control group.
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Real-life Examples
Ensrud, K. E., Stock, J. L., Barrett-Connor, E.,
Grady, D., Mosca, L., Khaw, K., et al. (2008). Effects of raloxifene on fracture risk in postmenopausal women: The raloxifene use for the heart trial. Journal of Bone and Mineral Research, 23(1), 112-120.
This research studied the effect of raloxifene on fracture risk in postmenopausal women, and found that the women who took raloxifene over the same five year period of time as the women who did not reduced their risk of clinical vertebrate fracture.
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Krishna, S., Balas, E. A., Francisco, B. D., & König, P. (2006). Effective and sustainable multimedia education for children with asthma: A randomized controlled trial. Children's Health Care, 35(1), 75-90.
This article reports on the effect of multimedia education for children with asthma. A control group of pediatric patients with asthma was given standard asthma educational resources, while the experimental group of pediatric patients with asthma was given standard resources plus multimedia resources. The study found a reduction in daily symptoms, in emergency room visits, in school days missed, and in days of limited activity in the group given multimedia education resources.
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Secondary studies
1-systematic reviews; which uses an explicit and reproducible method to review primary studies.
1-meta-analysis; which is a mathematical synthesis of the result of two or more primary studies.
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All Rights Reserved @ 2009 Deryck Pattron, Ph.D. 46
Meta-Analyses
Systematic objective way to combine data from many studies
Pooled estimate of treatment effectiveness
Statistical significance
Publication bias
Theoretical framework
Misleading results
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All Rights Reserved @ 2009 Deryck Pattron, Ph.D. 47
Systematic Reviews
Comprehensive survey
All relevant studies of highest level of evidence
Rigorous analysis
Systematic review
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Summary points;
Many papers published in medical journals have potentially serious methodological flaws.
When deciding whether a paper is valid and relevant ,first establish what specific question it address.
Question about therapy should be addressed by clinical trials.
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Summary points;
Questions about prognosis require cohort study.
Diagnosis: demonstrating whether a new diagnostic test is valid and reliable. preferred study is cross sectional survey.
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Conclusion (1)
Case-series is a coherent set of cases of a disease (or similar problem).
Cases are compared with reference group, we have a case control study
In a population studied at a specific time and place (a cross-section) the primary output is prevalence data, though association between risk factors and disease can be generated.
In cross-sectional studies, we are looking for both exposure and outcome
In case-control studies, we know the outcome, looking for the exposure
In cohort studies, we know the outcome, following up looking for the outcome in question
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Conclusion (2)
If the population in a cross-sectional survey is followed up to measure health outcomes, this study design is a cohort study.
If the population of such a study are, at baseline, divided into two groups, and the investigators impose a health intervention upon one of the groups the design is that of a trial.
Studies based on aggregated data are commonly referred to as ecological studies.
RCTs represent the “gold standard” of research designs. They thus provide the most convincing evidence of relationship between exposure and effect..
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SCENARIO 1
What are the risk factors for the development of sarcoidosis?
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SCENARIO 2
What are the long-term effects of the daily use of topical minoxidil?
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Scenario 3
Is there a difference between pediatricians and family
physicians in the practice of neonatal circumcision?
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Scenario 4
Does cigarette smoking cause lung cancer?
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THANKS FOR YOUR KIND ATTENTION AND LISTENING