| Summary: | Accessing structured data in the form of ontologies currently requires the use of formal query languages (e.g., SeRQL or SPARQL) which pose significant difficulties for non-expert users. One way to lower the learning overhead and make ontology queries more straightforward is through a Natural Lan- guage Interface (NLI). While there are existing NLIs to structured data with reasonable performance, they tend to require expensive customisation to each new domain. Additionally, they often require specific adherence to a pre-defined syntax which, in turn, means that users still have to undergo training. In this thesis, we study the usability of NLIs from two perspectives: that of the developer who is customising the NLI system, and that of the end-user who uses it for querying. We investigate whether usability methods such as feedback and clarification dialogs can increase the usability for end users and reduce the customisation effort for the developers. To that end, we have developed two systems, QuestIO and FREyA, whose design, evaluation and comparison with similar systems form the core of the contribution of this thesis.
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