This type of study is used to indicate relationships between nutritional habits, disease trends and other health phenomena of large populations. it is called

observational study

Explanation:

The observational studies are those studies where the researchers observes the samples or individuals of research without any intervention or manipulation.

In epidemiology, and nutritional studies the researcher observes the risk factors associated with the disease, diagnosis and treatment. The effect of a diet on the patient or volunteers can be done through observational studies.

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