https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-20 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-22 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-26 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-17 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-27 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-16 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-18 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-24 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-28 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-13 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-15 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-21 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-19 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-23 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-25 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-13-14