Sample size estimation based on GEE requires several parameters, all of which were unknown at the design stage of the study. These included the correlation of physicians’ recommendations for TKA, specifying the structure of the correlation matrix, and a scale factor allowing for extra variation (or dispersion) in the response beyond the assumed variance [12]. For these reasons, although we planned to analyze our data using GEE, we based our sam-ple size of h 5 58 on what was considered to be a