Operating a quality assurance system in tertiary education is the rule rather than the exception, because of the belief that it will improve quality. However, proving this is not easy. This study examines three ways of providing the evidence: the a "priori" method, the stepwise backtracking method, and the external evaluation method. The quality assurance system of the Vocational Training Council of Hong Kong is used as a case-study, but the findings on the advantages and disadvantages of using these methods have relevance for testing the effectiveness of quality assurance systems for other education institutions. (Contains 2 figures, 2 tables, and 5 notes.)