## 1 Introduction  
Open educational resources (OER) have a huge and irrefutable potential to provide universal and equal access to education at all levels, contributing to education's democratization [1]. This potential has been noticed by the European Union, who recognized that stimulating supply and demand for high-quality OER is essential for modernizing education and that these resources contribute to broadening access to education, as well as to alleviating costs for educational institutions and students, especially among disadvantaged groups [2]. The first definition of the OER term was proposed at a UNESCO conference in 2002 which defined OER as \"the open provision of educational resources, enabled by information and communication technologies, for consultation, use, and adaptation by a community of users for non-commercial purposes\" [3]. A definition more often used nowadays states that OER are \"digitized materials offered freely and openly for educators, students, and self-learners to use and reuse for teaching, learning, and research\" [4]. The educational benefits of OER most frequently recognized include the encouragement of improvement and localization of content, the drastic reduction of costs without detriment to instructional effectiveness, and the capacity to offer equal access to knowledge for all. The current OER panorama reveals that there is an enormous and growing amount of digital learning resources available for learners and teachers through OER repositories. Repositories such as Wikimedia Commons [5], Europeana [6], OER Commons [7], MERLOT [8], ODS [9], or the LRE [10] are offering thousands or even millions of OER. For example, Wikimedia Commons offers more than 61 million freely usable media files, whereas Europeana makes available nearly 59 million digitized materials from European museums, galleries, and multimedia archives. This massive amount of resources available in OER repositories makes it difficult for users to find relevant and high-quality content when searching using these systems. A recent systematic literature review [11] indicated that discoverability of quality open learning resources was one of the major barriers hindering OER adoption and impact. According to the results of the OER Data Report 2013-2015 [12], finding resources of sufficiently high quality and related to a specific subject area are challenges often experienced by both educators and learners when searching for OER. Other studies have also pointed out difficulties in locating relevant and quality educational resources. [PERSON] et al. [13] investigated usability problems of search tools for repositories of educational resources and concluded that these tools were not easy to use. [PERSON] et al. [14] explored the search services offered by eight of the most popular OER repositories and pointed out gaps, as well as possible solutions. Another study [15] showed that teachers could require more than an hour to find a suitable learning resource, and that this search process was perceived in many occasions to be time consuming and occasionally frustrating. An aspect that deserves special attention in OER repositories is the pedagogical quality of the learning resources, as many of these repositories follow a production model in which OER are created by communities of volunteers and lack an effective quality control mechanism. These are the main reasons why quality assurance is still often pointed out as an open issue for OER [16; 17; 18; 19]. There are two main reasons why quality assurance is especially important for OER repositories. On the one hand, teachers need to have some guarantee of quality before including OER in the syllabus. Two surveys [12; 20] have provided evidence that lack of quality control has been a problem for teachers when using OER authored by others and that user ratings were important for teachers when searching for OER.