one list of words. if you’re doing a within design where you’re testing memory with and without interference, then you can’t use the same list of words because of practice effects. you need two (or more) lists of words, but you also need to make sure that the lists are equally difficult to remember, so that the only explanation for any difference you find is that for one list there was interference.
in general, when you’re doing a study of memory for word lists where you’re using more than one list of words, then you need to design two matched word lists that you can show are equivalent on any possible 