Diagnosing Bias vs. Variance

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Diagnosing Bias vs. Variance

In this section we examine the relationship between the degree of the polynomial d and the underfitting or overfitting of our hypothesis.

  • We need to distinguish whether bias or variance is the problem contributing to bad predictions.
  • High bias is underfitting and high variance is overfitting. Ideally, we need to find a golden mean between these two.

The training error will tend to decrease as we increase the degree d of the polynomial.

At the same time, the cross validation error will tend to decrease as we increase d up to a point, and then it will increase as d is increased, forming a convex curve.

High bias (underfitting): both Jtrain(Θ) and JCV(Θ) will be high. Also, JCV(Θ)Jtrain(Θ).

High variance (overfitting)Jtrain(Θ) will be low and JCV(Θ) will be much greater than Jtrain(Θ).

The is summarized in the figure below:

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