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).