Spine
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Retrospective cohort study. ⋯ 3.
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Retrospective analysis of prospective data. ⋯ 4.
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This study is based on data from the 2009-2012 National Health Interview Survey (NHIS) of the civilian population of the United States. The NHIS focuses on a number of health conditions, including low back pain. ⋯ 2.
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Retrospective comparative study. ⋯ 3.