Is it ethical to use data you overheard in a conversation? I do research on particular elites of a particular society and rely heavily on interview data. In one fortuitous dinner, at the table behind me sat three people who are exactly the people I wanted to get access to.

Furthermore, they were discussing something that I am precisely investigating! I was attentive to their conversation (as they were, honestly, speaking very loudly) but by no means eavesdropping and their conversation - if viewed in the context of my work - can prove to be very valuable.

Can I use that data for my work? It does not seem ethical to me. But, if I do not use that data, I know I will be 'deliberately' weakening my research and not being true to what I do now know.

Thoughts? What would you do and what would you suggest I do?
Without understanding your research topic and methods, it is hard to say objectively if you can use the data. That said, the American Psychological Association (APA) classifies naturalistic observation as exempt from the informed consent process. This does not mean it does not require approval by an ethics board (i.e., an IRB). Naturalistic observation studies are valid in a wide range of fields including Medicine, Psychology, Zoology and Ethnography. That said, as with any research methods, it takes skills to conduct and interpret naturalistic observation studies properly and within the framework of their limitations. This seems like a nice starting point: http://peace.saumag.edu/faculty/kardas/Courses/RMPA/naturalisticobservation.html