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Ruf KC, Fehn S, Bachmann M, Moeller A, Roth K, Kriemler S, Hebestreit H.Ruf KC, et al.BMC Med Res Methodol. 2012 Apr 3;12:43. doi: 10.1186/1471-2288-12-43.BMC Med Res Methodol. 2012.PMID: 22471343Free PMC article.