MERLOT
MERLOT (Multimedia Educational Resource for Learning and Online Teaching) is a catalogue of vetted teaching materials rather than a dataset you download. Curators and an international community of educators review and index tens of thousands of online learning objects, lessons, simulations, and open textbooks across every academic discipline, layering peer reviews, ratings, and member-built collections on top.
The key thing to grasp is that MERLOT mostly points elsewhere. Each catalogue entry is a description and a link out to material that lives on someone else's site. You browse and search through the web interface, and there is no bulk export or open API for the underlying content, so building a corpus from it means following the links and retrieving each target yourself.
For RAG, MERLOT earns its place as a discovery layer: a way to find high-quality, human-reviewed educational content on a topic before you commit to ingesting anything. It suits teams building tutoring or curriculum assistants who want curated, reviewed material rather than whatever a raw web crawl happens to turn up.
The caveats are those of any pointer catalogue. Because the resources are hosted externally, licence terms vary from one item to the next, so you must check the terms on each resource before reusing it rather than assuming a single blanket licence. Links rot over time, and what you find at the far end (HTML, PDF, an interactive applet) is inconsistent, so extraction takes real work.
It sits alongside OER Commons and the Mason OER Metafinder in the open educational resources discovery toolkit. Use MERLOT and those tools to locate content, then handle the ingestion, extraction, and per-item licence checking yourself.
Related sources
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AGIEval
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CASE Network (1EdTech)
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CEFR Companion Volume Descriptors
The Council of Europe's 2020 CEFR Companion Volume descriptor set, the de facto international standard for levelling language proficiency from Pre-A1 to C2. A spine of can-do statements for sequencing and grading language teaching.