Which researcher used both overt participant observation and questionnaires to study a religious group?

The choice of Barker as the correct answer rests on their unique approach in researching religious groups. Barker conducted an in-depth study of a religious community, specifically focusing on the Moonies, by employing both overt participant observation and questionnaires. This methodology allowed Barker to gather rich qualitative data through direct involvement and interaction within the group, while also obtaining quantitative data through structured questionnaires. This combination provided a comprehensive understanding of the group's beliefs and practices from both insider and outsider perspectives.

The other researchers listed did not utilize this specific combination of methods in their studies of religious groups. Willis, for example, is known for his work on working-class boys and education, utilizing participant observation but not specifically focusing on religious groups. Linda Hart's work has mainly been focused in other fields rather than religious studies, and Chamberlain and Gillborn have approached different sociological phenomena, focusing on issues such as race and education, without the same methodological application as Barker. Thus, the empirical methodology employed by Barker aligns perfectly with the requirements of the question regarding the study of a religious group.