Which term most accurately describes the following definition? "Analysis based on aggregated or grouped data; errors in inference may result because associations may be artifactually created or masked by the aggregation process." [Porta M,ed.A Dictionary of Epidemiology.New York: Oxford University Press; 2014.]

A) Ecologic study
B) Cross-sectional study
C) Case-control study
D) Cohort study
E) Randomized trial

Ecologic studies are studies of group characteristics to determine whether an association exists."Cross-sectional study" is not correct.Cross-sectional studies define a population and determine the presence or absence of exposure and the presence or absence of disease for each individual at the same time.Both exposure and disease outcome are determined simultaneously for each subject."Case-control study" is not correct.In case-control studies,investigators identify a group of individuals with the disease and a group of individuals without the disease to examine the possible relation of an exposure to a certain disease."Cohort study" is not correct.In cohort studies,investigators select a group of exposed individuals and a group of nonexposed individuals and follows up both groups to compare the incidence of disease in two groups."Randomized trial" is not correct.A randomized trial is a clinical-epidemiologic experiment in which subjects are randomly allocated into groups,usually called test and control groups,to receive or not to receive a preventive or a therapeutic procedure or intervention.