1. When looking for data, consider who’d go to the trouble of collecting & using it, and why.
Example 1: If seeking vital health statistics (e.g. births, deaths), remember that the U.S. Government is mandated to collect these data on a regular basis through the U.S. Census.
Example 2: If seeking data on a particular ethnic population's access to local HIV/AIDS counseling services, search within local community organizations, non-profits etc.
2. It’s quite possible that the data you seek is not available online (especially if it’s many years old), but you may still be able to request it via another library, or else from the originating data-gathering organization.
3. Available data might not correspond to your geographic area or demographic group of interest.
4. Identify the original source of the data you're using (the organization responsible for its collection).
5. For any set of data you use, pay attention to any associated scope notes, limitations, annotations, exceptions or other qualifications that have been placed on them—those qualifications may help explain differences/discrepancies between the results of different studies and surveys.
6. Surveys, data collection, analysis and reporting are time-consuming, and there's usually a significant delay (e.g. months or years) before published data become available.
7. Implementing health-related surveys is costly and labor/expertise-intensive. It's often only local or national government organizations--including those that are mandated to collect such data--who have access to the resources needed to implement such surveys. Smaller organizations and non-profits are likely to collect data on more focused/specific geographic or subject scope, depending on their specific goals and purposes.
8. Remember that you can also identify data sources through the references cited in the relevant research literature--including articles you'd find in databases like PubMed, CINAHL, ProQuest.