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Study design in Epidemiology
Kristen ReyherCentre for Veterinary Epidemiological Research
Atlantic Veterinary CollegeUniversity of Prince Edward Island
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The theory behind study design
• We want to design studies to– Identify causal factors for disease,
so that we can• Focus on target points to work towards prevention • Minimise harmful effects of treatments or
management changes
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Study types
Descriptive Explanatory (Analytical)
Case Report
Case Series
Survey Observational Experimental
Cross-sectional
CohortCase-
controlLaboratory
Controlled Trials
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Descriptive vs. Analytical
• Descriptive– Describe
characteristics– Do not make
comparisons• case report• case series• survey
• Analytical (explanatory)– Seek to make comparisons
• inference about exposures (risk factors, treatments) and outcomes (disease, death, production)
– Experimental vs. observational
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Descriptive studies 1
• Case study– Report on one or a few cases
– Usually a rare condition– Limited to ‘real world’ conditions?
– Any conclusions about cause or outcome are author’s conjecture
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Descriptive studies 2
• Case series– Describe (often unusual) clinical course of
condition of interest– Might provide information about prognosis if
cases are representative of all cases
– Again, no direct data but features might help build hypotheses
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Descriptive study 3
• Survey– Estimate the frequency and distribution of
outcomes– Provides some data (say about disease in a
population)
– Need to take care re: sampling (Signe) and design of questions
– Surveys including exposures and outcomes = cross-sectional analytic studies!
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Analytical studies
• Experimental– Investigator can allocate study subjects
– Advantages• stronger evidence of causation• control of confounders through randomisation
– Disadvantages• limited range of hypotheses• may not be “do-able”
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Analytical studies
• Observational– No allocation of study subjects
• Do not confuse random sampling with random allocation!
• Observation in a real-world setting
– Advantage• Complex web of causation might not be otherwise
reproducible– practically– ethically– economically
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Observational studies
• Prospective vs. Retrospective– Has outcome occurred before study starts?
• yes = retrospective• no = prospective
– Advantage of prospective• data quality• better able to study incidence
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Observational studies
• Classified by subject selection– Cross-sectional studies
– Cohort studies – Case-control studies
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Observational study 1
• Cross-sectional– Most frequent study design in vet epi =
straightforward– Random sample of subjects from a population
• Try to represent population in sample
– Non-directional in time = ‘snapshot’– Simultaneously classify according to
• Disease status (or outcome)• Study factor or risk factor
– determinant– exposure
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Cross-sectional
Time
Past Present Future
F+/D+
F+/D-
F-/D-
F-/D+Study Population
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Cross-sectional studies
• Limitations– Only suitable for chronic conditions occurring at a
moderate level in the population– Only quantifies prevalence of exposure and outcome
• May over-represent factors affecting incidence and duration• Can confuse protective risk factors
– Reverse-causation• Best for time-invariant exposures (sex, breed, housing)• Can confuse procedures implemented in response to disease
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Cross-sectional studies
• Example: – 100 dairy herds selected randomly from
the Dairy Herd Improvement (DHI) register to answer a questionnaire assessing treatment of dry cows
– Each farm is classified according to exposure (which dry cow therapy used) and according to outcome (>30% fresh cow mastitis or <30% fresh cow mastitis)
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Cross-sectional
Time
Past Present Future
F+/D+
F+/D-
F-/D-
F-/D+100 farms
Used DryClox / >30% mastitis
Used DryClox / <30% mastitis
Used CefaDri / >30% mastitis
Used DryClox / <30% mastitis
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Cross-sectional studies
• Pros– Representative of population– Potentially efficient– Low cost– Rapid
• Cons– Must verify that risk factor came before
the disease
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Observational study 2
• Cohort– Identify subjects
• with exposure• without exposure
– Follow the groups through time to determine if disease develops
• usually prospective
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Cohort
Time
Past Present Future
non-exposed
exposed
D+
D-
D+
D-
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Cohort studies –special case
• Single cohort = longitudinal study– Follows an entire population through time– Record all exposures of interest
• Investigate multiple exposures at once
– Record all outcomes of interest• Outcomes must follow exposure!
– Useful in measuring incidence of disease
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Cohort Studies
• Example: – 500 healthy cows (D-) from farms using dry
cow therapy (E+) were randomly selected from a list of cows in DHI databases across Canada
– 500 healthy cows (D-) from farms not using dry cow therapy (E-) were also randomly selected from the same list of cows
– Followed for two years– Assessed for clinical mastitis during that time
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Cohort
Time
Past Present Future
Farms not using dry
cow therapy
Farms using dry
cow therapy
Mastitis
No mastitis
Mastitis
No mastitis
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Cohort studies
• Pros: – Less susceptible to bias compared to case-control– More control over quality of data– No confusion on time order of exposure and disease
• Cons:– Expensive– Time-consuming– Potential losses to follow-up– Only works for diseases common in a population
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Observational study 3
• Case-Control– Identify subjects
• with disease• without disease
– Compare histories of risk factor (exposure)• Usually retrospective
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Case-control
Time
Past Present Future
D+
D-
exposed
non-exposed
exposed
non-exposed
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Case-control studies
• Used for rare diseases• Relatively quick and inexpensive
(if quality data is accessible)
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Case-control studies
• Limitations– Finding source of cases
– Defining a case– Appropriate controls are often difficult to
identify. These should be animals that would have been cases if they got the disease (but not always as straightforward as that sounds!)
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Case-control• Example:
– A rare mastitis is being studied.
– 50 farms in Quebec has confirmed cases of this type of mastitis (D+)
– 50 comparable farms in Quebec with no confirmed cases (D-) are also identified for the study
– All 100 farmers are asked about management practices (type of dry cow therapies) used on their farms (exposure)
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Case-control
Time
Past Present Future
50 farms with
mastitis
50 without mastitis
Used CefaDri
Used DryClox
Used CefaDri
Used DryClox
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Case-control studies
• Pros:– Rare diseases
– Potentially efficient– Low cost
– Potential for rapid completion
• Cons:– Highly susceptible to bias related to
selection of controls
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Summary of observational studies
Study type Cross-sectional Cohort Case-control
Minimal cost � �
Short time (little to no follow-up)
� �
Control selection difficult�
Representative of population
�
Good for rare disease �
Good for rare exposure �
Time sequence known �
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Experimental studies
• Laboratory-based• Randomised controlled trials
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Experimental study 1
• Laboratory-based– Carried out under strictly controlled conditions
– Investigator has almost complete control over experimental conditions
– Evidence of association of exposure and outcome is the best evidence of causation
– Relavance to ‘real-world’ conditions often doubtful
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Experimental study 2
• Randomised controlled trials– Covered by Signe next!
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Characteristics of various study types
Type of studyLevel of
difficulty
Level of investigator
control
Strength of “proof” of
causal association
Relevance to “real-world”situations
Descriptive
Case report very easy very low n/a low to high
Case series easy very low n/a low to high
Survey moderate moderate n/a high
Explanatory - experimental
Laboratory moderate very high very high low
Controlled trial moderate high very high high
Explanatory - observational
Cross-sectional moderate low low moderate
Cohort difficult high high high
Case-control moderate moderate moderate high
n/a = not applicable34