General
- determine whether there is an association between factor / characteristic and development of disease
- derive appropriate inferences regarding possible causal relationship from patterns of association
Design of a Cohort Study
- investigator selects group of
o exposed individuals
o non exposed individuals
- incidence : a/ (a+b)
- example
o coronary heart disease CHD
§ smokers à exposed / non smoker à not exposed
§ both groups followed for development of CHD
§ incidence in both groups compared.
- identify new cases
o can determine new cases à can determine whether a temporal relationship exists between exposure and disease
o temporal relationship must be established if we are to consider exposure possible cause
Comparing Cohort Studies with Randomized Trials
- difference
o presence / absence of randomization
- without randomization
o when we observe an association
o left with uncertainity
§ association may be result of fact that people were not randomized to exposure
§ not the exposure, but rather factors that led people to be exposed that are associated with diseae
§ example
· increased risk of disease in workers at factory
· most workers live in certain area
· exposure
o place of work or residence
Selection of Study Populations
- two ways
o create study population by selecting groups for inclusion on bais of whether or not exposed.
o select defined population before any members become exposed (or before exposure identified)
§ select based on factor not related to exposure
· community
Types of Cohort Studies
- prospective cohort study
o also
§ concurrent cohort / longitudinal study
o issues
§ time involved
- retrospective cohort
o also
§ historical cohort study
§ non current prospective study
o use historical data to ascertain exposure
o outcome (disease / no disease) is known at time study is begun.
Examples
- The Framingham Study
o cardiovascular disease
o many exposures were defined.
o second method of selection
§ selected on residence
o cohort study that begins with exposed / non exposed group -> focuses on specific exposure
o cohort study that begins with defined population à can explore role of many exposures
- Incidence of breast cancer and progesterone deficiency
Cohort Studies for Investigating Childhood Health and Disease
- exposure dose was calibrated on bais how far person was from point of bomb drop
- at what point should individuals in cohort be first identified
- sample from one centre / several clusters / nationally
- how long should cohort be followed / changes in knowledge etc
Potential Biases in Cohort Study
- bias in assessment of outcome
o can be fixed by masking
- information bias
o if quality and extent of info obtained is different for exposed and non exposed persons
- bias from non response and loss to followup
- analytic bias
When is a cohort study warranted
- when we have good idea of which exposures are possible causes of disease
- need to minimise attrition
- historical records not available to do retrospective cohort study à case control
- disease has low rate of occurrence.
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