improving music mood classification using lyrics, audio and social tags

5.2.2.4 Negative Samples
Figure 5.1 Example of labeling a song using social tags
In a binary classification task, each category needs negative samples as well. The negative
sample set for a given category are chosen from songs that are not tagged with any of the terms
found within that category but are heavily tagged with many other terms. Since there are plenty
of negative samples for each category, a song must satisfy all of the following conditions to be
selected as a negative sample:
1) It has not been tagged by any of the terms in this category;
2) The total normalized counts of all tags that are not in this category is no less than 100;
3) The minimum normalized count among all tags associated with this song is 0 or 1.
Condition 2) and 3) together make sure the total absolute count of “other” tags is no less,
and probably much more than 100.
Similar to positive samples, all negative samples have at least 100 words in their unfolded
lyric transcripts. For each category, the positive and negative set sizes are balanced, and thus the
total number of examples in all categories is 12,980.
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