## 3 Results
\begin{table}
\begin{tabular}{c c c c c c} \hline  & & **Onsite 1** & **Onsite 2** & **Online 1** & **Online 2** \\ \hline \hline \multirow{2}{*}{\begin{tabular}{c} n \\ \end{tabular} } & \multirow{2}{*}{52} & 31 & 23 & 30 \\ \hline \multirow{2}{*}{\begin{tabular}{c} Age (median (IQR)) \\ \end{tabular} } & \multirow{2}{*}{29.5 (25, 35.25)} & 33 (28, 35.5) & 34 (28.5, 4.5) & 25.00 (22.25, 32.75) \\ \hline \multirow{2}{*}{\begin{tabular}{c} Sex (\%) \\ \end{tabular} } & Female & 17 (32.7) & 7 (22.6) & 8 (34.8) & 12 (40) \\  & Male & 35 (67.3) & 24 (77.4) & 15 (65.2) & 18 (60) \\ \hline \multirow{2}{*}{\begin{tabular}{c} Study\_years\_starting (median (IQR)) \\ \end{tabular} } & \multirow{2}{*}{4 (1, 9.25)} & \multirow{2}{*}{8 (4, 12)} & \multirow{2}{*}{4 (4, 4)} & \multirow{2}{*}{10.00 (5.25, 13.75)} \\ \hline \multirow{2}{*}{\begin{tabular}{c} Postgraduate\_training (\%) \\ \end{tabular} } & Expert & 19 (56.5) & 7 (22.6) & 1 (4.3) & 1 (3.3) \\  & Master & 16 (30.8) & 17 (54.8) & 0 (0) & 0 (0) \\  & PhD & 0 (0) & 0 (0) & 0 (0) & 1 (3.3) \\  & Weekend courses & 17 (32.7) & 7 (22.6) & 16 (69.6) & 21 (70) \\  & NA & 0 (0) & 0 (0) & 6 (26.1) & 7 (23.3) \\ \hline \multirow{2}{*}{\begin{tabular}{c} Previous\_US\_training (\%) \\ \end{tabular} } & No & 35 (67.3) & 4 (22.9) & 22 (95.7) & 29 (96.7) \\  & Yes & 17 (32.7) & 27 (87.1) & 1 (4.3) & 1 (3.3) \\ \hline \multirow{2}{*}{\begin{tabular}{c} Type\_of\_US\_training (\%) \\ \end{tabular} } & Non regulated training & 8 (15.4) & 4 (12.9) & 1 (4.3) & 0 (0) \\  & None & 32 (61.5) & 3 (9.7) & 22 (95.7) & 29 (96.7) \\  & Regulated training & 12 (23.1) & 24 (77.4) & 0 (0) & 0 (0) \\  & Self-training & 0 (0) & 0 (0) & 0 (0) & 1 (3.3) \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} US\_training\_hours (\%) \\ \end{tabular} } & 15 to 20 & 7 (13.5) & 14 (45.2) & 1 (4.3) & 2 (6.7) \\  & 21 to 40 & 4 (7.7) & 6 (19.4) & 0 (0) & 0 (0) \\  & Less than 15 & 9 (17.3) & 1 (3.2) & 1 (4.3) & 3 (10) \\  & More than 40 & 0 (0) & 7 (22.6) & 1 (4.3) & 0 (0) \\  & None & 32 (61.5) & 3 (9.7) & 20 (87) & 25 (83.3) \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Hours\_week\_using\_US (\%) \\ \end{tabular} } & 1 to 4 & 13 (25) & 11 (35.5) & 2 (8.7) & 1 (3.3) \\  & 5 to 10 & 3 (58) & 9 (29) & 1 (4.3) & 2 (6.7) \\  & 11 to 20 & 0 (0) & 3 (9.7) & 1 (4.3) & 0 (0) \\  & More than 20 & 0 (0) & 1 (3.2) & 0 (0) & 0 (0) \\  & Not use & 36 (69.2) & 7 (22.6) & 19 (82.6) & 27 (90) \\ \hline \end{tabular} Data expressed as median and interquartile range (IQR) or with absolute and relative values (\%).  
\end{table} 
Table 1: Baseline characteristics of the sample. In the Hollow_structure, the model detected a small negative difference between the Online 1 group and the total effect (\(-0.9830\), SE = 0.4386) and a significant difference (t = \(-2.241\), \(p=0.0267\))  
Figure 1: Propensity scores by group. Figure 2: Standardized mean difference between inverse probability of treatment (IPTW) model and unweighted data. with a low effect size (R\({}^{2}\)adjusted = 0.08). In the Solid_structure scores the model detected a small negative difference between the Onsite 2 group and the total effect (\(-\)1.23148, SE = 0.43687) and a significant difference (t = \(-\)2.819, \(p\) = 0.00558) with a low effect size (R\({}^{2}\)adjusted = 0.025). In the Solid_structure_surface_distance the model detected a positive difference between the Online 2 and Onsite 1 groups (1.5026, SE = 0.5822) and a significant difference (t = 2.5809, \(p\) = 0.0113) with a low effect size (R\({}^{2}\)adjusted = 0.016). The results showed significant differences between some of the training groups and the outcome variables; however, it was confirmed that despite the weighted adjustment of the covariates, very large differences remained in the SMDs, above the tolerable limit of 0.2, both between the covariables and between the groups; in fact, between the groups with the least differences between them (Onsite 1 and Onsite 2) no significant differences were detected in any of the outcome variables.