Based on SMOTE, Safe-Level-SMOTE, Safe-Level-Synthetic Minority Oversampling
TEchnique, assigns each positive instance its safe level before generating
synthetic instances. Each synthetic instance is positioned closer to the largest safe
level so all synthetic instances are generated only in safe regions.
The safe level (sl) is defined as formula (7). If the safe level of an instance is close
to 0, the instance is nearly noise. If it is close to k, the instance is considered safe. The
safe level ratio is defined as formula (8). It is used for selecting the safe positions to
generate synthetic instances