DISSENYS D’ESTUDIS EPIDEMIOLÒGICS
Dr. Josep M. VilasecaCAPSE 2009
CROSS - SECTIONAL STUDY
Data are collected on each study participant at a single point in time. It can be descriptive analytical.
DESCRIPTIVE CROSS - SECTIONAL STUDY
collects information on the frequency and distribution of health-related exposures or outcomes in a defined population
The main epidemiological outcome measure obtained is therefore prevalence, either point prevalence or period prevalence
Bias: recall bias
ANALYTICAL CROSS - SECTIONAL STUDY
Are undertaken to investigate the association between exposure to risk factors and the outcome of interest. They require this information to be collected simultaneously on each individual
Recall bias is even more relevant Non-response can introduce a systematic bias Limitation of a.c-s.s.: it is not possible to show
causality
CROSS - SECTIONAL STUDY
Strenghts: Quick and easy to perform They can determine the prevalence of risk
factors and the frequency of prevalent cases
Useful for measuring current health status Repeated studies can be used to
determine changes in risk factors
CROSS - SECTIONAL STUDY
Weaknesses: Data on disease and risk factors are
collected simultaneously (problem to determine temporal relationship)
Include prevalent rather than incident cases, which might cause the study group to be biased toward patients with chronic ilness.
ECOLOGICAL STUDY
It is carried out at the population level. They use aggregate data and do not measure outcomes and risk factors in individual people:Multigroup studies (cross-sectional, to
compare different groups)Longitudinal (time-trend or time-series)
They can be descriptive or analytical
ECOLOGICAL STUDY
EcAny attempt to infer from the group level to the individual level – ecological inference – gives rise to the ecological fallacy (or bias)
Data on exposure and outcome may be collected in different ways with different definitions which may bias the results
COHORT STUDIES Also known as incidence studies,
longitudinal studies or follow-up studies Cohort: a group of individuals who share a
common characteristic The cohort is defined by its exposure to a
possible risk factor for the outcome under study
COHORT STUDIES
Prospective cohorts identify the participants and then follow them up over time until the end-point of interest (or time limit for the study) has been reached
Retrospective cohorts use pre-existing data on exposures and outcomes and therefore do not need to follow individuals over time since all the information is already available
COHORT STUDIES - BIASSES
Healthy worker effect Measurement (information) bias Observer bias Loss to follow-up
COHORT STUDIES - measures
Risk ratio Rate ratio (dichotomous variables) Atributable risk / rateThe observed association may be affected
by chance, bias and confounding