It's true that POS tagging works fairly well. But consider that a sentence involves more than one word. Even at 97 % accuracy for one word, the probability of correctly tagging every word in a short sentence of only ten words, is still as low as 0.97^10 = 0.74. And sentences are generally longer than ten words.
And as POS tagging is usually only done as preprocessing for some other task like syntactically parsing a text (which itself is usually preprocessing for yet another task), 97 % accuracy per word is not as good as it sounds. Parsers need to work with wrong data for every second or third sentence.
And as POS tagging is usually only done as preprocessing for some other task like syntactically parsing a text (which itself is usually preprocessing for yet another task), 97 % accuracy per word is not as good as it sounds. Parsers need to work with wrong data for every second or third sentence.