204.4.9 Model-Bias Variance Tradeoff

Sweet spot for Bias and Variance.
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Model Bias and Variance

  • Over fitting
    • Low Bias with High Variance
    • Low training error – ‘Low Bias’
    • High testing error
    • Unstable model – ‘High Variance’
    • The coefficients of the model change with small changes in the data
  • Under fitting
    • High Bias with low Variance
    • High training error – ‘high Bias’
    • testing error almost equal to training error
    • Stable model – ‘Low Variance’
    • The coefficients of the model doesn’t change with small changes in the data

The Bias-Variance Decomposition

Y=f(X)+ϵ Var(ϵ)=σ2 SquaredError=E[(Yf^(x0))2|X=x0]

= σ2+[Ef^(x0)f(x0)]2+E[f^(x0)Ef^(x0)]2

= σ2+(Bias)2(f^(x0))+Var(f^(x0))

Overall Model Squared Error = Irreducible Error + \(Bias^2\) + Variance

Bias-Variance Decomposition

  • Overall error is made by bias and variance together.
  • High bias low variance, Low bias and high variance, both are bad for the overall accuracy of the model.
  • A good model need to have low bias and low variance or at least an optimal where both of them are jointly low.
  • How to choose such optimal model. How to choose that optimal model complexity.

Choosing optimal model-Bias Variance Tradeoff

Bias Variance Tradeoff

Test and Training Error

Choosing Optimal Model

  • Unfortunately,there is no scientific method of choosing optimal model complexity that gives minimum test error.
  • Training error is not a good estimate of the test error.
  • There is always bias-variance tradeoff in choosing the appropriate complexity of the model.
  • We can use cross validation methods, boot strapping and bagging to choose the optimal and consistent model.

The next post is about cross validation.

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