“If your learner’s test-set accuracy disappoints, you need to diagnose the problem. Was it blindness or hallucination? In machine learning, the technical terms for these are bias and variance. A clock that’s always an hour late has high bias but low variance. If instead the clock alternates erratically between fast and slow but on average tells the right time, it has high variance but low bias. Suppose you’re down at the pub with some friends, drinking and playing darts. Unbeknownst to them, you’ve been practicing for years, and you’re a master of the game. All your darts go straight to the bull’s-eye. You have low bias and low variance, which is shown in the bottom left corner of this diagram:”