text: Four problems render vector space model -LRB- VSM -RRB- - based text classification approach ineffective : 1 -RRB- Many words within song lyrics actually contribute little to sentiment ; 2 -RRB- Nouns and verbs used to express sentiment are ambiguous ; 3 -RRB- [[ Negations ]] and modifiers around the sentiment keywords make particular contributions to << sentiment >> ; 4 -RRB- Song lyric is usually very short .