Abstract Background To illustrate how (standardised) effect sizes (ES) vary based on calculation method and to provide considerations for improved reporting. Methods Data from three trials of tanezumab in subjects with osteoarthritis were analyzed. ES of tanezumab versus comparator for WOMAC Pain (outcome) was defined as least squares difference between means (mixed model for repeated measures analysis) divided by a pooled standard deviation (SD) of outcome scores. Three approaches to computing the SD were evaluated: Baseline (the pooled SD of WOMAC Pain values at baseline [pooled across treatments]); Endpoint (the pooled SD of these values at the time primary endpoints were assessed); and Median (the median pooled SD of these values based on the pooled SDs across available timepoints). Bootstrap analyses were used to compute 95% confidence intervals (CI). Results ES (95% CI) of tanezumab 2.5 mg based on Baseline, Endpoint, and Median SDs in one study were - 0.416 (- 0.796, - 0.060), - 0.195 (- 0.371, - 0.028), and - 0.196 (- 0.373, - 0.028), respectively; negative values indicate pain improvement. This pattern of ES differences (largest with Baseline SD, smallest with Endpoint SD, Median SD similar to Endpoint SD) was consistent across all studies and doses of tanezumab. Conclusion Differences in ES affect interpretation of treatment effect. Therefore, we advocate clearly reporting individual elements of ES in addition to its overall calculation. This is particularly important when ES estimates are used to determine sample sizes for clinical trials, as larger ES will lead to smaller sample sizes and potentially underpowered studies. Trial Registration Clinicaltrials.gov NCT02697773, NCT02709486, and NCT02528188. standardised (standardised (ES analyzed (outcome mixed analysis (SD scores evaluated treatments treatments]) assessed assessed) timepoints. timepoints . timepoints) 95 CI. CI (CI) (95 25 2 5 2. 0416 0 416 0.41 ( 0796 796 0.796 0.060, 0060 0.060 , 060 0.060) 0195 195 0.19 0371 371 0.371 0.028, 0028 0.028 028 0.028) 0196 196 0373 373 0.373 respectively improvement largest Therefore Clinicaltrialsgov Clinicaltrials gov NCT02697773 NCT NCT02709486 NCT02528188 treatments] 9 (CI (9 041 41 0.4 079 79 0.79 006 0.06 06 019 19 0.1 037 37 0.37 002 0.02 02 NCT0269777 NCT0270948 NCT0252818 04 4 0. 07 7 0.7 00 0.0 01 1 03 3 0.3 NCT026977 NCT027094 NCT025281 NCT02697 NCT02709 NCT02528 NCT0269 NCT0270 NCT0252 NCT026 NCT027 NCT025 NCT02 NCT0