Professor Nakum designs a memory experiment to test the effect of word familiarity on memory. Three lists of words are created: common words, uncommon words, and made-up words. Participants are randomly assigned to study one of the lists of 30 words for 5 minutes, do math problems for 5 minutes, then write all the words they recall from the list. Their score is the number of words correctly recalled.

Required:
What is the independent variable in this study?