Week 3 Flashcards
descriptive cross sectional study
describe frequency of exposure or outcome in a defined population
analytical cross sectional study
collects information on both the outcome of interest and potential risk factors in a defined population
then compare the prevalence of the outcome in the people exposed to each risk factor with the prevalence in those not exposed
what does a cross sectional study do
estimate frequency or outcome at a particular point in time
why do a cross-sectional study
health service planning- prevalence of specific outcome in a defined population at point in time
useful for assessing burden of disease and planning preventative and curative services
main steps of a cross sectional study
- defining the study
- defining the target population
- selecting the study population
- collecting data
- analysing data
- interpreting results
survey sampling
can make statements about the population by asking a small sample
convenience samples
ie patients attending a clinic
is the group representative
random sample
each subject has equal chance
computer generate random numbers
stratified sampling
population divided into groups (based on similarities)
take random sample from each group
pitfalls in surveys
inaccurate data
non-coverage
non-response
analysis of cross sectional studies
prevalence ratio: prevalence of outcome in exposed
interpretations of cross sectional studies
true association or reverse causality
random error
bias
confounding
bias in cross sectional studies
selection bias: characteristics of those taking part vs those not taking part
information bias: recall bias
how to minimise bias in cross sectional studies
by having strict case definition for the outcome of interest
using standardised methods of data collection
ensuring that the researcher who assigns the diagnosis is blinded to exposure status
strengths of cross-sectional study
easy and economical
provides important information on the distribution and burden of exposures and outcomes- valuable for health-service planning
can be used as the first step in the study of a possible exposure-outcome relationship
weakness of cross-sectional studies
measures prevalent rather than incident cases
can be difficult to establish the time-sequence of events in a cross-sectional study
- the exposure may have occurred as a result of the outcome (reverse causality)
definition of ecological studies
observational study with populations or groups (instead of individuals) being unit of observation
what does ecological studies do
compare group averages
- health
- risk factors
interpretations of ecological studies
true casual (association) or reverse causality
random error
bias
confounding
uses of ecological studies
describes associations at group level
quick and cheap- routine data
generates hypotheses- first step
some risk factors may not easily be measurable at an individual level
what is ecological fallacy
ecological studies enable us to make ecological inferences about effects at the group level. They do NOT enable us to make inferences about individual risks
an attempt to infer from the ecological level to the individual level is often called ecologcil fallacy
