Predictors of surgical re-intervention: Variable importance measure (VIM)
To identify important predictors of surgical re-intervention we used two methods for analysis. First, a statistical univariate logistic regression analysis was applied to assess the importance of each variable. For each variable an odds ratio (OR) with a 95% confidence interval (CI) was calculated. Secondly, a permutation-based variable importance was used. This VIM is based on AUC statistic of the RF model. The AUC statistic is computed by randomly permutating the values of predictor x, and comparing the resulting AUC to the not permutated AUC. Leaving out an important feature will result in a lower AUC of the RF model, while leaving out an unimportant feature will not change the AUC significantly (23,38,41).