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This article has been retracted by the authors  because large portions of text have been duplicated from a number of previously published articles, including Tin et al., 2007  and Auyeung et al., 2009 . All authors agree with the retraction.
The online version of the original article can be found under doi:10.1186/1746-1596-8-179
Wenqiao Zang, Email: moc.anis@oaiqnewgnaz.
Guoqiang Zhao, Email: nc.ude.uzz@qgoahz.