# Question about the AIC tutorial

Hi,

Here, I get why you can drop the first term. But why the last term? Why is it a constant?

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The answer is that we don’t know $\sigma^2$ in advance, so we use the empirical estimate from the residual, but that’s just $\sigma^hat^2 = 1/N\sum_i^N (y_i - \tilde y_i)^2$. So the last term is a constant, it’s equal to n/2.