<s>[INST] context: Ecological study . Observational studies allow nature to take its own course; the investigator measures but does not intervene. Descriptive studies are those that investigate the distribution of disease in a population Analytical studies analyse the relationship between the disease and the causative factors. Experimental or interventional studies involve an active attempt to change a disease determinant or the progress of a disease (similar in design to experiments in other sciences) Descriptive studies Descriptive studies are usually the first phase of an epidemiological investigation. These studies are concerned with observing the distribution of disease or health-related characteristics in human population and identify the characteristics with which the disease in question seems to be associated. These studies provide clues to disease etiology and help in formulation of an etiological hypothesis. Case repo - the simplest of the descriptive studies is the case repo, which is often seen in medical journals and describes an unusual feature or unusual association between the disease and some exposure factor. Case series - a case series is a sequence of case repos with common elements such as similar clinical features and suspected common exposures. They can be valuable early evidence for association between exposures and diseases which can be studied in more detail subsequently; however as they lack a control (comparison) group, the case series can result in false leads, wasted energy & resources. Cross-sectional or Prevalence study - in cross-sectional studies, we 'survey' several individuals at one point in time and collect information on health status, health-related behaviours, and other exposure factors. In this way we can estimate the burden of disease in a group of individuals and try to estimate whether a paicular exposure is linked to a disease or health outcome. Ecological or correlational study - an ecological or correlational study is one in which the unit of analysis is an aggregate of individuals and information is collected on this group rather than on individual members. The association between a summary measure of disease and a summary measure of exposure is studied. E.g., There is a positive correlation between fat consumption and breast cancer across many nations. Conclusion: Fat consumption is a risk factor for breast cancer. An error of reasoning (Ecological Fallacy) occurs when conclusions are drawn about individuals from data that are associated with groups, as relationships observed for groups may not necessarily hold for individuals. In the above example, it is not possible to infer that the women with breast cancer actually had a high fat consumption. Analytical studies Analytical studies are the second major type of epidemiological studies. In contrast to descriptive studies that look at the entire population, in analytical studies the subject of interest is the individual within the population. The object is not to formulate, but to test hypotheses. Case Control - Sometimes it is necessary to study individuals in whom a disease has already occurred in order to find out whether these individuals had been exposed to a paicular risk factor. This is called a case control study. The cases are compared to a group of individuals who do not have the disease in question and the same information on risk factors is collected from both groups. By doing this, we may be able to find out if a paicular risk factor is more common among the cases than controls and therefore suggest an association between the risk factor and disease. Coho study - In a coho study, a group exposed to a paicular factor and another group not exposed to this factor are followed up over time to determine occurrence of disease. The incidence of disease in the exposed group is compared to the incidence of disease in the unexposed group. This is called the relative risk and it allows us to make a judgment on whether the exposure factor and the disease are causally linked. [/INST] question: A person found some correlation between dietary factors and a disease; he did this by collecting data from the food manufacturers and hospitals respectively, such a study is? | options: A: Ecological B: Cross sectional C: Psephological study D: Experimental study | answer: A </s>