The central idea of minimum description length analysis (Rissanen 1989) is composed of four parts: first, a model of a set of data assigns a probability distribution to the sample space from which the data is assumed to be drawn; second, the model can then be used to assign a compressed length to the data, using familiar information-theoretic notions; third, the model can itself be assigned a length; and fourth, the optimal analysis of the data is the one for which the sum of the length of the compressed data and the length of the model is the smallest.
this is a difficult task to accomplish well, as English illustrates, for we would like to be able to determine that NULL.s is primarily a subcase of 's.NULL.s, and not of (e.g.)