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Different Types of Epidemiologic Studies
Kamran Yazdani, MD MPHDepartment of Epidemiology and Biostatistics
School of Public Health
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CASP
Critical Appraisal Skills Programme
http://www.phru.nhs.uk/Pages/PHD/resources.htm(Appraisal Tools)
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CASP• Major Points:
• Is the Objective CLEAR?• Is the Method (Design & Analysis)
APPROPRIATE?• Dealing with:
• Selection Bias• Information Bias• Confounding• Chance error
• Reporting (Analysis, Results)• Interpreting (Association, Causation)• Generalizability• Consistency, Coherence
SC
RE
EN
ING
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STrengthening the Reporting of
OBservational studies in Epidemiology
http://www.strobe-statement.org/Checklist.html(Checklists)
STROBE
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Epidemiology
The study of the distribution and
determinants of health-related states or
events in specified populations, and the
application of this study to control of
health problems.
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Aim of epidemiological studies
•To determine distribution of disease
Descriptive Studies
•To examine determinants of a disease
To judge whether a given exposure causes or prevents disease
Analytical Studies
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انواع مطالعات
با هدف بررسي و توصيف يك توصيفي:
وضعيت بدون آنكه قصد بررسي يك رابطه
)آزمون فرضيه( را داشته باشيم
با هدف بررسي يك رابطه ، اختالف يا تحليلي:
ارتباط )آزمون فرضيه( صورت مي پذيرد
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Epidemiologic Design Strategies•Descriptive studies
•case reports, case-series, cross-sectional
•Analytical studies–Observational studies
•Cross-sectional•Case-control studies•Cohort studies
–Intervention studies •Clinical trials•Field trials•Community trials•Experimental (animals)
•Diagnostic Tests studies or Process Research
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انواع مطالعات
توصيفي تحليلي
مشاهده اياي مداخله
كارآزمايي باليني
كارآزمايي اجتماعي
كارآزمايي ميداني
مقطعي
مورد شاهدي
كوهورت
اكولوژيك
گزارش مورد
گزارش موارد
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Advantages of Case Reports and Case series
•Allows for the description of new disease processes.
•Allows for the description of outcomes associated with rare diseases or rare features of any disease.
•To formulate hypotheses of the association
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Disadvantages/Limitations of Case Report & Case Series
•Impossible to determine disease frequency.
•Cannot establish causality between exposures or risk factors and disease outcome.
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Case Report example
In 1961, a published case report of a 40 year-old woman who developed pulmonary embolism after beginning use oral contraceptive
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Case Series example
In Los Angeles, five young homosexuals men, previously healthy, were diagnosed with pneumocyst cariini pneumonia in a 6-month period (80-81)
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Case Series example
RESULTS: Twelve patients with histopathologically confirmed tumours detected after extraction of teeth were studied. There were 11 males and one female giving a male to female ratio of 11:1. They ranged in age from 15-85 years with a mean age of 53 years. Pain and swelling were the most common presenting complaints. The mandible was more often involved seven (58.3%) cases while five (41.7%) cases occurred in the maxilla. Squamous cell carcinoma (in 9 cases) was the most common malignant neoplasm among these patients.
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•Cross-Sectional Studies measure existing disease and current exposure levels.
•They provide some indication of the relationship between the disease and exposure or non-exposure
Cross-sectional studies
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Cross Sectional Studies (contd)
•Sample without knowledge of Exposure or Disease
•Sample at one point in time
•Mostly prevalence studies/surveys
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•Good design for hypothesis generation
•Can estimate overall and specific disease
prevalence and sometimes rates
•Can estimate exposure proportions in the
population
• Can study multiple exposures or multiple
outcomes or diseases
Cross Sectional Studies(Advantages)
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•Relatively easy, quick and inexpensive!!!
•Best suited to studying permanent factors (breed,
sex, blood-type) to deal with TEMPORALITY.
•Often good first step for new study issue
Cross Sectional Studies(Advantages)
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• Impractical for rare diseases• Not a useful type of study for establishing
causal relationships• Confounding is difficult to control• Problems with temporal sequence of data• hard to decide when disease was actually
acquired• miss diseases still in latent period• recall of previous exposure may be faulty
Cross Sectional Studies(Disadvantages)
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dc
ba
Yes No
Disease Status
Yes
No
Exposure Status
a +b
c +d
b +da +c N
Total
Cross Sectional Studies
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7514
873
Yes No
Depression
Yes
Nolow SES
90
89
16217 179
Total
Cross Sectional Studies
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Cross Sectional Studies
• Points need concern:
• Sampling• Protocol (Quality Assurance & Control)• Appropriate Analysis• Appropriate and Fair Interpretation
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Randomized Clinical Trials
New Treatment
Comparison treatment
Outcome
Participants
(Randomization)
Improved Not improved
Improved Not improved
Population
Inclu & Exclu
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7525
8713
Yes No
Cure
A
B
Treatment100
100
16238 200
Total
Randomized Clinical Trials
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Cohort / Follow-up Studies
Study population(Non-diseased)
Exposed
Non-exposed
Disease +
Disease +
Disease -
Disease -
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Cohort study, at a glanceCase control Cohort
Study group Diseased/ healthy
Exposed/ unexposed
Measure of effect
OR, AR Risk, RR,OR.AR
temporal Hard to establish Easy to establish
multiple exposures outcomes
time Short Long
cost inexpensive Expensive
Population size
small Large
Information bias
exposure Outcome
Best when D rare E frequent E rare D frequent
Problems Control selection
Exposure information
Unexposed selection
change over time
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جدول توافقي در مطالعه ي كوهورت
dc
ba
Yes No
Disease Status
Yes
No
Exposure Status
a +b
c +d
b +da +c
Total
n1
n2
N
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آيا مطالعه كوهورت هميشه تحليلي است؟
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مواجهه پي آمد
مواجهه پي آمد
مواجهه پي آمد
زمان حال
مطالعه كوهورت ـ انواع
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Prospective vs. retrospective Cohort Studies
Prospective Cohort Studies– Time consuming, expensive– More valid information on exposure– Measurements on potential confounders
Retrospective Cohort Studies– Quick, cheap– Appropriate to examine outcome with long latency
periods– Admission to exposure data– Difficult to obtain information of exposure– Risk of confounding
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مطالعه كوهورت چه موقع؟مناسب تر است
شواهدي موجود باشد••LOSS TO FOLLOW UP را بتوانيم كنترل
كنيممدت پي گيري نسبتا5 كوتاه باشد•بتوانيم كوهورت تاريخي انجام دهيم•مواجهه نادر باشد•
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Selection of the Exposed Population
Sample of the general population:Geographically area, special age groups, birth cohorts (Framingham Study)
A group that is easy to identify:Nurses health study
Special population (often occupational epidemiology):
Rare and special exposure
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Selection of the Comparison Population
• Internal Control Group– Exposed and non-exposed in the same Study
population (Framingham study, Nurses health study)
• Minimise the differences between exposed and non-exposed
• External Control Group– Chosen in another group, another cohort
(Occupational epidemiology: Asbestosis vs. cotton workers)
• The General Population
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Bias• Selection bias:
– Non-representativeness (unequal in e+ & e-)– Non-response during data collection– Losses to follow up– Healthy worker effect
• Information bias– Misclassification on exposure– Misclassification on event
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CASP questions
Adobe Acrobat 7.0 Document
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Case-control study
Study Population
Cases
Controls
Exposed
Non-exposed
Exposed
Non-exposed
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Direction
exposure outcome
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Applications
• Diseases with long latency period
• For best use of time and money
• The best for rare diseases & useful for prevalent diseases
• Mutiple exposures
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Case-control study, at a glanceCase control Cohort
Study group Diseased/ healthy
Exposed/ unexposed
Measure of effect
OR, AR Risk, RR,OR.AR
temporal Hard to establish Easy to establish
multiple exposures outcomes
time Short Long
cost inexpensive Expensive
Population size
small Large
Information bias
exposure Outcome
Best when D rare E frequent E rare D frequent
Problems Control selection
Exposure information
Unexposed selection
change over time
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BIASes
• Selection
• Information
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Selection of cases
• Case definition is more important than other studies:
– Strict diagnostic criteria (high degree of caseness)
– Homogenous diseases (one well-defined outcome or health-related state)
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Selection of cases
• Sources of cases– Population– Hospital (available, better dx, low inf bias,
high sel bias)– Registry– …
• Are the cases representative of total population or a fraction of it?
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Selection of cases
• Incident vs. Prevalent– Selection & Information BIASes
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Selection of controls
• Study base– Sel bias
• Deconfounding– confounding
• Comparable accuracy– Info bias
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Selection of controls
سوال:•آيا مي توان براي يك گروه بيمار كه از –
بيمارستان انتخاب شده اند، گروه كنترل را از جامعه گرفت؟
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Types of controls
• Population controls– Friend, RDD, neighbor controls
• Hospital controls– Similar disease as controls
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Hospital controls
• Similar study base
• Similar quality of information
• Convenience
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Population controls
• Tax lists, vote lists, telephone directories,…
• RDD: – Selection bias due to higher ses., families with
more than phone lines and family size
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Matching
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Ratio of controls to cases
• Statistical considerations (increasing power) – Unsuitable for very low or very high powers
under equal sizes
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Ratio of controls to cases
• Control to case ratio: up to 4-fold:
case cont
• 1:1 200 200
• 1:2 150 300
• 1:3 133 400
• 1:4 125 500
• 1:5 120 600
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جدول توافقي در مطالعه ي
مورد ـ شاهدي
dc
ba
Yes No
Disease Status
Yes
No
Exposure Status
a +b
c +d
b +da +c
Total
n1 n2 N
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CASP questions
Adobe Acrobat 7.0 Document
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OR و RRمقايسه
مي تواند برآورد ORدر مطالعه كوهورت • باشد اگر:RRخوبي از
بيماري بروز بااليي نداشته باشد• مي تواند ORدر مطالعه مورد شاهدي •
باشد اگر:RRبرآورد خوبي از بيماري بروز بااليي نداشته باشد•موردها نماينده بيماران جامعه باشند•شاهدها نماينده سالمهاي جامعه باشند•
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Comparing cohort and case controlCase control cohort
Study group Diseased/ healthy
Exposed/ unexposed
Measure of effect
OR, AR Risk, RR,OR.AR
temporal Hard to establish Easy to establish
multiple exposures outcomes
time Short Long
cost inexpensive Expensive
Population size
small Large
Information bias
exposure Outcome
Best when D rare E frequent E rare D frequent
Problems Control selection
Exposure information
Unexposed selection
change over time
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Intuition and Logic in Research
Dominant Mental ActivityIntuition
Feeling
Judgement
Experience
Analysis
Experiment
Control over variance
Hi
Potential for Misinterpretation
Qualitative
Research
Case Report
Case Series
Cross-sectional Study
Case-control Study
Cohort Study
Clinical trials
Lo
LoHi
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