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LOL. 15 million. 5% of the U.S. population!! I guess they don't mind lying to make a point.
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They do provide a source:
http://govinfo.library.unt.edu/ngisc/reports/4.pdf
The 15m at risk seems to be based on a telephone survey of 2417 supplemented by interviewing 530 people in gambling establishments to "increase the sample size of potential problem gamblers".
The "problem" level they have set is 2 from ten of the DSM-IV questions, the weakest three of which are "gambles to escape" which translates to me as gambles because you enjoy it. "Lies to family members re the extent" which translates as telling granny that you are fine and whether at any point in your lifetime you have increased the level of stake to achieve the "desired" excitement which to me means saving up for a big tournament.
If you applied these criteria to golf players or sports fans what result would you get? Yes Yes yes. Other surveys listed set the "problem" level of loss at $100 in a day.
To put it into some perspective the paper does include other lifetime problems:
Alcohol 23.5%, Drug dependence 11.9% and Gambing at 5.4%
PS this research is 1997 which makes it an odd choice for analysing online gambling.