This is like online reviews. If you selectively take positive or negative reviews and somehow censor the rest, the reviews are worthless. Yet, if you report on all the ones you find, it's still useful.
Yes, I'm more likely to leave reviews if I'm unsatisfied. Yes, people are more likely to leave CS if they were unhappy. Biased data, but still useful data.
Yes, I'm more likely to leave reviews if I'm unsatisfied. Yes, people are more likely to leave CS if they were unhappy. Biased data, but still useful data.