We consider the problem of predicting which words a student will click in a vocabulary learning system. Often a language learner will find value in the ability to look up the meaning of an unknown word while reading an electronic document by clicking the word. Highlighting words likely to be unknown to a reader is attractive due to drawing his or her attention to it and indicating that information is available. However, this option is usually done manually in vocabulary systems and online

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... opedias such as Wikipedia. Furthurmore, it is never on a per-user basis. This paper presents an automated way of highlighting words likely to be unknown to the specific user. We present related work in search engine ranking, a description of the study used to collect click data, the experiment we performed using the random forest machine learning algorithm and finish with a discussion of future work.