https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-168 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-176 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-171 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-177 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-170 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-172 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-174 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-180 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-178 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-182 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-167 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-169 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-173 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-175 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-181 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-179 https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/1471-2288-12-183