 Calculations of sample size Migration RSA. Several published RSA studies are available but none evaluating an uncemented Bi-Metric stem including data for the most clinically relevant migration between 1 and 2 years. Quite a lot of RSA studies evaluating migration of both cemented and uncemented femoral stems have been published, but most investigators give the variability of their RSA results as total range or standard error of mean. We have decided for our sample size calculation to use the average standard deviation SD -for the migration after 2 years of follow-up from two previously published studies including data from one uncemented n22 and two cemented stems n82, where clear information regarding SD was available in the publication. 