This study examined the rate at which English vocabulary was acquired from the 3 input modes of reading, reading-while-listening, and listening to stories. It selected 3 sets of 28 words within 4 frequency bands and administered 2 test types immediately after the reading and listening treatments, 1 week later and 3 months later. The results showed that new words could be learned incidentally in all 3 modes, but that most words were not learned. Items occurring more frequently in the text were more likely to be learned and were more resistant to decay. The data demonstrated that, on average, when subjects were tested by unprompted recall, the meaning of only 1 of the 28 items met in either of the reading modes and the meaning of none of the items met in the listening-only mode, would be retained after 3 months.
Brown et al. (2008) studied this question.