One important data source in public health is surveys. For example, each year a representative sample of New Yorkers answer numerous questions about their health for the Community Health Survey.
Other surveys focus on specific populations: the Youth Risk Behavior Survey looks at public high school students, while the World Trade Center Health Registry follows a large group of people who were exposed to the 2001 terrorist attack on New York City.
All these surveys are based on self-report, unlike the more costly NYC Health and Nutrition Examination Survey which asked participants to take a physical exam, providing more objective data for a small sample of New Yorkers.