Webb, E. J. (2000). Unobtrusive measures (Rev. ed.). Sage.
Lua i Stíl Chicago (17ú heag.)Webb, Eugene J. Unobtrusive Measures. Rev. ed. Thousand Oaks, Calif: Sage, 2000.
Lua MLA (9ú heag.)Webb, Eugene J. Unobtrusive Measures. Rev. ed. Sage, 2000.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.