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• Human intuition is harder to count on.​ For example, pretty much no one can
predict the stock market. So if our stock prediction algorithm does no better than random
guessing, it is hard to figure out how to improve it.

• It is hard to know what the optimal error rate and reasonable desired error
rate is. ​Suppose you already have a book recommendation system that is doing quite
well. How do you know how much more it can improve without a human baseline?

Page 67 Machine Learning Yearning-Draft Andrew Ng