Minimum Description Length Learning

MDL Learning is basically another interpretation for MAP Learning where we interpret the log terms

log⁡2(P(d∣hi))+log⁡2(P(hi))\log _2\left(P\left(d \mid h_i\right)\right)+\log _2\left(P\left(h_i\right)\right)

similar to how we did with Information Gain in Decision Tree Induction, as the number of bits to encode data given the hypothesis

−log⁡2(P(d∣hi))-\log _2\left(P\left(d \mid h_i\right)\right)

and the additional bits to encode the hypothesis

−log⁡2(P(hi)).-\log _2\left(P\left(h_i\right)\right).

So to get the best hypothesis hMDLh_{\text{MDL}} we have to minimize the Entropy of the hypothesis Likelihood.

arg⁡min⁡hi−log⁡2(P(d∣hi))\underset{h_i}{\arg\min}\quad -\log _2\left(P\left(d \mid h_i\right)\right)

Intuition
If the hypothesis predicts the data exactly, then P(d∣hi)=1P(d\mid h_i)=1 which yields log⁡2(1)=0\log_2(1)=0 bits. So this is the preferred hypothesis.