How does a book determine if you're a sharp?

[bills217]

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I really know nothing about this, I originally thought a book would only adjust someone's lines if they were a big winner, betting a large amount per game etc, but a friend of mine has different lines than I do, and he bets very small, so do they just change them whenever you go a winning streak?

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Bodog, for example, shades toward faves to take advantage of the public. If you bet mostly/entirely their artificially inflated dog lines, they start dealing you their sharp (aka normal) lines. Otherwise they are printing money for you, whether you bet a little or a lot. Other books probably work along the same lines - it isn't rocket science.