This one is still quite pertinent. I often recommend it as a staring point to learn about Cognitive Flexibility Theory. It forms a foundation with a few other papers that can be found here: The Spiro, Coulson, Feltovich & Anderson paper from the 1988 Proceedings of the Cognitive Science Society and the 1990 Spiro & Jehng paper from the Nix & Spiro book, Cognition, Education & Multimedia.
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