In the beginning, Web search was based on aspects of the resources being indexed, like term frequency and inverse document frequency. That's of course still important but things really took off when search engines started to treat the Web as a connected graph, placing as much importance on the interconnectedness of the resources as what was in the resources themselves. Google made a huge jump forward in this area with its PageRank algorithm.

As things stand, search for digital material for teaching and learning (or learning objects or open educational resources) focuses on the resources themselves (or metadata about them), just as early search engines did. These resources are usually stored separately from the systems in which they are used, in disconnected repositories.

Erik Duval and his team are doing some interesting work in this area with what they call attention metadata. It's a good start, but until we do away with repository silos and start using the system of use as the repository, this area of search will move slowly. If this is done, there will be all sorts of other information to use in heuristics to optimise results.