Analytical epidemiological studies. Cohort studies - types, design, conducting, advantages and disadvantages.
Cohort studies:
type of analytical study to obtain additional evidence to support the existence of an association between suspected cause & disease
Concept:
group of people who share a common characteristic within a defined time period (e.g. age, occupation, etc) —> i.e. group of people born on the same day / in same period of time
comparison group: general population from which the cohort is drawn or another cohort of persons that have little or no exposure to the substance in question
Design:
Nur kopiert aus Vorlesungen, da ich es nicht verstehe: In contrast to case-control studies which proceed from effect to cause the basic approach in cohort study, exposure & disease have already occurred when the study is initiated. In a cohort study the exposure has occurred but the disease has not.
Begin: group or cohort (a+b) exposed to particular factor thought to be related to disease occurrence & group (c+d) not exposed to that particular factor
But: cohort must be free from the disease under study, groups should be equally susceptible to the disease under study, groups should be comparable in respect of all possible variables
- Selection of study objects: subjects of cohort study assembled from general population or select groups of the population that can be readily studied (i.e. persons with different degrees of exposure to the suspected causal factor)
- Obtaining data on exposure: from cohort members, review of records, environmental surveys+
- Follow-up: at start of study methods should be devised depending upon the outcome to be determined to obtain data for assessing the outcome —> periodic medical examination of each member of the cohort, reviewing physician & hospital records, routine surveillance of death records
Analysis- estimation of risk:
Relative risk (RR): ratio of the incidence of the disease (or death) among exposed & incidence among non-exposed
Attributable risk (AR): difference in incidence rates of disease (or death) between an exposed group & non-exposed group
Advantages:
offer possibility of studying full range of effects of the suspected etiological factor
directly measure incidence rates within exposed & non-exposed persons
provide direct estimates of the risk of disease for each exposure group separately
enable to study rare expositions
less subject to bias —> exposure is evaluated before health status is known
Disadvantages:
time consuming, expensive logically difficult
not suitable for studying rare diseases