C: decision directed acyclic graph (DDAG) method
In DDAG method, classes are the same except one at decision
nodes of each level, namely, there are many overlapping pattern
classes at the level nodes except leaf nodes. Therefore, more support
vectors are needed to separate two subgroups, which badly
affects the training and testing speed. In this way, N×(N−1)/2
child classifiers of SVM should be obtained, without nonseparable
region. In addition, different choices of root node will give birth to
different result for a decided class, which leads to uncertain classification
result.