Παραπομπή σε μορφή APA (7η εκδ.)

Helal, A. H., & Weiss, J. W. (1993). Opportunity 2000: Understanding and serving users in an electronic library. Universitatsbibliothek Essen.

Παραπομπή σε μορφή Chicago (17η εκδ.)

Helal, Ahmed H., και Joachim W. Weiss. Opportunity 2000: Understanding and Serving Users in an Electronic Library. Essen: Universitatsbibliothek Essen, 1993.

Παραπομπή σε μορφή MLA (8th εκδ.)

Helal, Ahmed H., και Joachim W. Weiss. Opportunity 2000: Understanding and Serving Users in an Electronic Library. Universitatsbibliothek Essen, 1993.