the items i j−1 j−m ) which hasn't been observed during training, a back-off to the corresponding reduced order chain is done 3 : Note that if the 0-th order chain also hasn't been observed during training, we are effectively backing off to the uniform distribution.
a caveat: the training dataset for an author can be much smaller (and hence have a smaller vocabulary) than the combined training dataset for the generic author, resulting in when a previously unseen chain is encountered there is a dangerous bias towards author a, i.e., Kernel based techniques, such as SVMs, allow the comparison of, and discrimination between, vectorial as well as non-vectorial objects.