Lecture 6- ecological and cross-sectional studies

experimental studies

investigator assigns exposure; has control over who is exposed to a factor of interest, participants are randomly assigned to the treatment vs. placebo group by the investigator, biological mechanism

observational studies

investigator does not assign exposure, has no control over who is exposed to a factor of interest, focus on health and disease status, exposure-outcome relationships, ex. ecologic and cross-sectional studies

ecologic studies

at least one variable, either the exposure or the outcome, is measured at the group level; units of analysis are populations or groups of people rather than individuals

population level exposure

per capita meat intake, average levels of air pollution in an area, average sunlight exposure in a region

measure of association

correlation between exposures and outcomes is studied, correlation coefficient (r) is the measure of association

ecological fallacy

inaccurate inferences drawn about individual level relationships based on aggregate data

ecologic studies- advantages

may provide information about the context of health, can be performed when hypothesis is new, can be performed when individual-level measurements are not available, can be conducted rapidly and with minimal resources

ecologic studies- disadvantages

ecologic fallacy, imprecise measurement of exposure/outcome, cannot adjust the measure of association for individual level factors

cross-sectional studies

a study that examines the relationship between exposures and outcomes based on individual level data collected at the same point in time, exposures and outcomes measured at the same time

prevalence study

current exposure status in study population (prevalence of exposure) and current disease status in the study population (prevalence of disease)

NHANES and BRFSS

use cross-sectional study design, tell us about prevalence of a number of factors in the population, can use these surveys to assess exposure-outcome relationships

cross-sectional studies and causality

provide weak evidence of causality primarily because the exposure and outcome are assessed simultaneously, can’t be certain that exposure preceded the disease

cross-sectional studies- advantages

useful for generating hypotheses that can be confirmed in analytical studies, inexpensive and simple, useful for planning and resource allocation, can study multiple exposures and outcomes at the same time

cross-sectional studies- disadvantages

exposure and outcomes assessed simultaneously, if the exposure/outcome is very rare not efficient, not useful for assessing incidence or the risk of developing a disease