Then again, I assume those texts are written by humans for humans. So isn't the "correct" sentiment exactly what humans tend to make of it? And if humans aren't very good at detecting the sentiment, maybe the writer is at fault, not the readers.
I think letting a number of people read the text and choosing the majority vote as the text's sentiment might not actually be a very bad way of determining that.
It might be correct to say that a group of humans is interpreting the sentiment "incorrectly" if they don't have all the relevant context / information.
I think letting a number of people read the text and choosing the majority vote as the text's sentiment might not actually be a very bad way of determining that.