Since the instances belonging to the three "no relation" classes significantly outnumber those belonging to the remaining eight classes, we reduce data skew by downsampling  instances belonging to the three "no relation" classes.
2 we downsample the instances belonging to the three "no relation" classes (i.e., NTrP, NTeP, and NPP) by ensuring that (1) the ratio of the number of NTrP instances to the number of TREATMENT-PROBLEM instances is 0.06; (2) the ratio of the number of NTeP instances to the number of TEST-PROBLEM instances is 0.03; and (3) the ratio of the number of NPP instances to the number of PROBLEM-PROBLEM instances is 0.5.