Studies of exposure are often observational designs. Observational studies are more likely to require difficult statistical adjustments because of the potential for large covariate imbalances. Since the researchers did not control who became seropositive, this is an observational study. These women were divided into low, intermediate, and high groups on the basis of their consumption of dietary fat. Since the women themselves controlled their diets, rather than having a diet imposed on them by the researchers, this represents an observational design.
Parallel groups design .
Both groups received the normal standard of medical care. The patients did not know which group they were assigned to, and the outcomes were measured in a blinded chart review. A collection of items of interest in research. The population represents a group that you wish to generalize your research to. Populations are often defined in terms of demography, geography, occupation, time, care requirements, diagnosis, or some combination of the above .