# Interactive 3D Exploration of RDF Graphs through Semantic Planes
## 3 Use Cases
### Use Case #2: Exploring DBpedia
It is then possible to notice, in the specific example, that the predicate is linked to the ground by a high number of links, so many other statements may have this predicate; the project only has three links with the ground (a click on them reveals that are links of type foaf:currentProject). In Figure 16, another example based on the same knowledge base is proposed. All the statements have been moved to a proper semantic plane (Step a), while one of the organizations (i.e., instances of foaf:Organization) was moved to the topmost plane (Step b). Then, all the rdf:type edges have been hidden for the sake of readability. This allows visually identifying the relationship among the selected organization and all the instances of rdf:Statement. It is then easy to notice how this organization took the burden of acknowledging all of the present statements. Figure 14: Use Case #3: The unfiltered knowledge base of the reification use case. Figure 15: Use Case #3: A statement has been moved to a dedicated plane. ## 4 Evaluation

In this section, the preliminary results of the evaluation of the user experience are presented (Section 4.1), followed by a computational assessment of the performance of the tool (Section 4.2). ### User Evaluation

Since Tarsier was mainly born as a tool to support students dealing with Semantic Web technologies, an evaluation of the user experience is important to assess the validity of the approach and identify possible improvements. Sixteen participants were selected among the students attending the course \"_Interoperability of Embedded Systems_\" held at the Computer Engineering faculty of the University of Bologna. Six of them have had previous minor experience on Semantic technologies. For the others, this university course has been the first contact with RDF knowledge bases. User evaluation methods based on a set of tasks to assess efficiency and effectiveness were employed. The evaluation was preceded by a short presentation of the tool (10 min) where the basics of the UI and the aim of the tool were discussed. Then, the experimenters were free to explore the tool and perform a feature walkthrough (10 min). Through the concurrent think-aloud protocol (CTA), we gathered the insights of users' cognitive processes while both free experimenting and performing five assigned tasks characterized by increasing complexity. In the first three tests, they were asked to interact with a local SPARQL Endpoint based on SEPA [13], the same adopted by students for their final assessment project. The remaining tests were about the visualization of data contained in DBpedia. Participants were free to allocate the desired amount of time to carry out the assigned tasks for a total time of one hour. At the end of the test, students completed a survey with two set of questions: the first task-specific, intended to understand how the user carried out each task, while the second aimed at the overall evaluation of the tool. Students were also allowed to write a short sentence after each question to express their opinion and highlight strong and weak points of the tool and its approach based on semantic planes. The test was intended to assess the level of usability and the learning curve, the overall feedback and the perceived level of utility and novelty. This was achieved through ten five-point Likert items (selected scale: _Strongly disagree_, _Disagree_, _Neutral_, _Agree_, and _Strongly Agree_). The results of the final users questionnaires are shown in Figure 17. The preliminary results suggest that the tool is useful for newcomers, as well as effective to understand data. The idea of semantic planes and the filtering mechanism were judged positively by all the participants. Among the suggestions received from the students, the most common was related to \"adding more visual tips and feedbacks\". This request was promptly accepted and new messages to confirm the action performed by the user were added.