
???

What else makes this an insufficient measure of error?

Positive and negative residuals would cancel each other out. We can fix that by taking the absolute value of each residual...

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class: middle, center

# Quiz

What could go wrong?

$$ \frac{1}{n} \sum_{i=1}^n |\hat{y}_i - {y}_i|$$ 

.footnote[Mean Absolute Error]

???

...but absolute values are hard to work with mathematically. They're not differentiable at zero. That's not a big deal to us because we can use computers. But it mattered in the