Language ENvironment Analysis
'LENA', dubbed by the press as a "verbal pedometer", is a small, expensive device that you can stick in a baby's pocket (you can also buy a jumper with a pocket 'specially designed to hold the device!); once there it will count the words spoken to, and by, the baby. It runs about $400, and it is worthless.
This is sold under the theory that the speed of a baby's speech development depends on its exposure to speech. More speech means quicker development. This is backed by research: higher word count is correlated with faster language development. (And higher parent earnings, FWIW). However, correlation does not equal causation!
Word count is absolutely meaningless in the absence of social interaction. It appears that what actually boosts an infant's development of language is the parents' responsiveness to the child making an utterance. When the parent responds quickly and appropriately, the baby learns that 1. words are important to us and 2. what mommy is saying has something to do with what mommy is waving in my face.
A 'verbal pedometer' is only theoretically useful if you only turn it on when you are playing directly with your baby. Even so, it won't tell you how quickly or how appropriately you respond. You are probably better off putting that money towards a baby sitter who really enjoys their job; they are likely to be playing with the baby just the way they are supposed to. Odds are, you're doing it right yourself, unless you've gotten caught up in reading about some nutty parenting fad.
Tamis-LeMonda, C.S., Bornstein, M.H., Maternal responsiveness and early language acquisition, Advances in Child Development and Behavior, vol. 29, pp. 89-127 (2002).
Tamis-LeMonda, C.S., Bornstein, M. H., & Baumwell, L. (2001). Maternal Responsiveness and Children's Achievement of Language Milestones, Child Development, 72 (3), 748-767.
If you must know: LENA