Boxplots of TSS values computed on 10 test sets for the prediction of C1+ class flares (top panel) and M1+ class flares (bottom panel) by successively excluding data samples of different classes from the test sets. Top panel. From left to right: (1) all classes are present in the test sets; (2) NO3 samples are excluded from the test sets; (3) NO3 and NO4 samples are excluded from the test sets; (4) NO3, NO4, and NO2 samples are excluded from the test sets. Bottom panel. From left to right: (1) all classes are present in the test sets; (2) NO3 samples are excluded from the test sets; (3) NO3 and NO4 samples are excluded from the test sets; (4) NO3, NO4, and NO2 samples are excluded from the test sets; (5) NO3, NO4, NO2, and C class samples are excluded from the test sets.