Download - 4. Cohort
KNOWLEDGE FOR THE BENEFIT OF HUMANITYKNOWLEDGE FOR THE BENEFIT OF HUMANITY
PUBLIC HEALTH AND EPIDEMIOLOGY (HFS3063) Epidemiological Study Designs:
COHORT STUDY
Dr. Dr. MohdMohd RazifRazif ShahrilShahril
School of Nutrition & Dietetics School of Nutrition & Dietetics
Faculty of Health SciencesFaculty of Health Sciences
UniversitiUniversiti Sultan Sultan ZainalZainal AbidinAbidin
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Topic Learning Outcomes
By the end of this lecture, students should be able to;
• describe cohort study design.
• explain the advantages and disadvantages of cohort study design.
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Cohort studies • A group of subjects, selected to represent the population
of interest, is studied over time.
• Information is collected about the outcome of interest
and exposure to risk factors.
• Subjects are disease-free at the outset of the study.
• At distinct point of time, data are collected relating to
health outcomes and exposure to risk factors.
• Used to examine causal factors.
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Prospective vs. Retrospective Cohort
• Prospective cohort studies
– Studies carried out from present time into future.
– Can be tailored to collect specific exposure data.
– But long wait for events to occur.
– Expensive.
– Prone to high dropout rates.
• Retrospective (historical) cohort studies
– Look at medical events from some timepoint in the past up to the
present time.
– Advantage: Information available immediately.
– Disadvantage: (1) difficulty in tracing subjects, (2) reliance on the
memory of subjects and/or the quality of recorded information
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(cont.) Prospective vs. Retrospective Cohort
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Selection and follow-up of subjects
• The ‘healthy entrant effect’
– a disease free status on entry to the study
– Initially, subjects are seen to have lower levels of disease than
might be true of the population in general
• with an acceleration of disease rate over time
• Following cohorts from birth may overcome ‘healthy
entrant effects’
– May result in lengthy and costly study
• Follow-up of subjects is carried out to monitor changes in
health status over time.
• Loss to follow-up will increase with the length of study.
– Plan mechanism to achieve lowest possible dropout rate.
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(cont.) Selection and follow-up of subjects
• Systematic differences between those who drop out
and those who stay in the study
– related to the outcome or exposure to risk factors
• Analysis of data must include a comparison of risk
factors between individuals who remain in the study and
those who have dropped out.
• If loss to follow-up is ignored
– Reliability of study conclusion can be questioned.
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Inferring causality
• To infer causality with any degree of certainty
– experimental study design
• Longitudinal nature of cohort studies
– Enables the assessment of causal hypotheses
– It is known if exposure occurred prior to outcome
• Measuring changes in levels of exposure over time
alongside changes in outcome measure gives
information on
– Dose response relationship between exposure and outcome .
• Higher level of exposure, associated with higher levels of
outcome
– provide further argument for causality.
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Hypotheses of cohort studies
Straightforward comparison of two groups, those with
exposure and those without.
• Ho: on average, the disease in the exposed group is no
different to that of the unexposed group
• E.g.: comparing cancer occurrence among those who
smoke with those who do not.
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(cont.) Hypotheses of cohort studies
When there are more than two groups.
• E.g.: The smoking status are split into those who do not
smoke, those who smoke fewer than 20 cigarettes a day
and those who smoke over 20 cigarettes.
• Type of grouping in strata, it has three hypotheses.
– Ho: on average, the outcome rates in groups of subjects are no
different to that of the population as a whole
– Ho: on average, the outcome rates in groups of subjects are no
different from each other
– Ho: on average, there is a linear trend in outcome rates across
groups
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Analysis of data
• Statistics from cohort study;
– Crude rates of outcome
– Standardized rates and ratios of outcome
– Risk ratio of outcome
• Crude Rates
– Number of individuals with the outcome out of the total cohort
study size
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(a + b) / n
(cont.) Analysis of data
• Standardized Rates and Ratios
– Can be calculated using demographic information so that rates
of the outcome are adjusted for other potential risk factors such
as age and sex
• Risk Ratio
– The risk of the outcome in exposed subjects relative to those not
exposed
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[a / (a + c)] / [ b / b + d)]
Advantages of cohort studies
• The temporal dimension, whereby exposure is seen to
occur before outcome, gives some indication of
causality.
• Can be used to study more than one outcome.
• Good for the study of rare exposures.
• Can measure the change in exposure and outcome over
time.
• Incidence of outcome can be measured.
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Disadvantages of cohort studies
• Costly (less so for retrospective) and may take long time – Particularly where onset of the outcome measure can occur both
early and late on in life.
• Require accurate records for retrospective studies.
• When studying rare outcomes, a very large sample size is required.
• Prone to dropout.
• Changes in etiology of disease over time may be hard to disentangle from changes observed as age increases.
• Selection bias – A difference in incidence of the outcome of interest, between
those who participated and those who did not, would give biased results.
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Recapitulate In this lecture, you have been exposed to;
• definition and types of cohort study .
• selection and follow-up of subjects.
• inferring causality
• hypotheses of cohort studies.
• analysis of data.
• advantages of cohort studies.
• disadvantages of cohort studies.
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Thank YouThank You
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