Triangulation is the use of two or more research methods in a single piece of research to check the reliability and validity of research evidence.
Triangulation is useful because not only does it check the research you have done, it can also offer new insights and ways at looking at things, especially if using very different types such as official statistics and observations.
There are many examples of triangulation. For example in his study The Tearoom Trade Humphries used questionnaires, unstructured interviews and participant observation.
Similarly in her study of the Moonies, Barker used participant observation, interviews and questionnaires.