# Bias items and probability estimates in LibSVM

I have two questions in using LIBSVM

1. The decision function for C-support vector classification is

$$\text{sgn}\left(w^T\phi(x)+b\right)=\text{sgn}\left(\sum_{i=1}^ly_i\alpha_iK(x_i,x)+b\right)$$

How can I understand the physical meaning of $b$? How can I enable LIBSVM to output these two types of information, $w$ and $b$?

2. LIBSVM is able to output probability estimates. When reading the articles of SVM, I also read similar term like confidence? I am not sure whether they are talking the same thing?

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