This study provides both a quantitative estimate and qualitative analysis of the additional ‘retrieval power’ that professionally assigned subject indexing affords users of a typical database in the field of education. A full version of Informit’s A+ Education database and one stripped of its subject indexing were searched by four research assistants tasked with compiling exhaustive bibliographies on forty-eight topics. The searchers were then surveyed about their use of the two databases, while their bibliographies and search logs were also examined. A two-way ANOVA model was constructed to estimate the percentage of additional resources found by the searchers on the full version of the database, while the survey responses, bibliographies and logs were analysed qualitatively. It was estimated that the subject indexing increases the yield of relevant resources by an average of between 48.1 and 124%, across the four searchers and all possible topics. The qualitative analysis showed that the indexing and controlled vocabulary was used by searchers in different ways, and that it also provided them with more selection power. The study provides clear evidence that professional indexing and controlled subject vocabularies can greatly enhance the performance of those tasked with scholarly literature searches.