make use of two-part codes: the first part always represents the information that one is trying to learn, such as the index of a model class (model selection) or parameter values (parameter estimation); the second part is an encoding of the data given the information in the first part. The difference between the methods is that, in the MDL literature, it is advocated that unwanted parameters should be moved to the second part of the code, where they can be represented with the data by using a so-called one-part code, which is often more efficient than a two-part code. In the original description of MML, all parameters are encoded in the first part, so all parameters are learned. Within the MML framework, each parameter is stated to exactly that precision, which results in the optimal overall message length: the preceding example might arise if some parameter was originally considered "possibly useful" to a model but was subsequently found to be unable to help to explain the data (such a parameter will be assigned a code length corresponding to the (Bayesian) prior probability that the parameter would be found to be unhelpful). In the MDL framework, the focus is more on comparing model classes than models, and it is more natural to approach the same question by comparing the class of models that explicitly include such a parameter against some other class that doesn't. The difference lies in the machinery applied to reach the same conclusion. See also Algorithmic probability Algorithmic information theory Inductive inference Inductive probability Lempel–Ziv complexity References Further reading Minimum Description Length on the Web, by the University of Helsinki. Features readings, demonstrations, events and links to MDL researchers. Homepage of Jorma Rissanen, containing lecture notes and other recent material on MDL. Advances in Minimum Description Length, MIT Press, . Algorithmic information theory