Bland-Altman plots. Comparison of the difference between the two methods of measurement (ETS and goniometer) versus the average of the two methods, for femoroacetabular impingement patients (•) and healthy subjects (°). Systematic bias is given by the solid line. Limits of agreement are given by the ± 2SD limits. A) Hip flexion. B) Hip abduction. Note that modified limits of agreement (with equations) are shown for hip flexion, as data revealed proportional bias.