For a given passage, 1. one human subject guessed the target word based on the whole passage (comprising the context and the target sentence); if the guess was right, 2. a second subject guessed the target word based on the whole passage; if that guess was also right, 3. more subjects tried to guess the target word based on the target sentence only, until the The subjects in step 3 were allowed 3 guesses per sentence, to maximize the chances of catching cases where the target words were guessable from the sentence alone.
although co-reference seems to play a big role, only 0.3% of target words are pronouns.