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| First time |  31% |
 30% |
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| More than once a day | 10% |
| 6% |
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| Daily | 13% |
| 13% |
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| About once a week | 25% |
| 30% |
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| About once a month | 17% |
| 15% |
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| Every 6 months or less | 5% |
| 6% |
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| Copyright | Privacy | Accessibility | Quality Guidelines U.S. National Library of Medicine, 8600 Rockville Pike, Bethesda, MD 20894 National Institutes of Health | Department of Health & Human Services |
Date last updated: 23 April 2007 |