This article has been retracted. Dr Songming Huang has advised that although he agreed to be listed as the correspondence author he should not have been named as a co-author of this publication because he made no contributions to it.
He and Song Mao have asked for the article to be retracted on the ground that data included in it and its conclusions are to a significant degree the same as those previously published in Chin J Evid Based Pediatr 2011;6:4.
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