When somebody mentions "X is a $Y million market", where are these numbers coming from? For example, in an article recently linked on HN:
http://www.xconomy.com/boston/2009/09/11/techstars-first-class-of-boston-startups-launched-at-microsoft-hosted-gala/
"Baby monitors are a $350 million market in the US"
Is this a wild-assed guess, or are these sorts of things tracked and tabulated somewhere? When looking to start a new business, where do hackers look to discover the size of their target markets?