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 c

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.