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