The Recent Letter " Language, Trees and Zipping " [1] Suggests Using Standard
compression programs to solve a number of problems. Unfortunately, the ideas are well known, and the technique, tested on a standard problem, is at least a factor of three worse than a simple baseline. In particular, the ideas of this Letter are very well known in several fields of Computers Science, including Machine Learning and Statistical Natural Language Processing. This Letter is essentially showing several examples of text classification, a well studied area in both fields. Bayesian models of many sorts have already been applied to text classification, and Bayesian reasoning is equivalent to the MDL principal used in this Letter [2, page 463-464]. For a good overview, read, for instance, the Text Categorization chapter of Manning and Schütze[4] (a standard introductory textbook) where numerous Bayesian and other approaches are described. In addition, the zipping algorithm described here was invented by Rich Caruana in 1995, and described in a tutorial by Tom Mitchell in 1997 [5]. Elsewhere [3], I give a broader description of the large body of related work. It is unfortunate that the authors did not try any of the standard algorithms used in Computer Science for problems of this type, or try their algorithm on any of the standard data sets. For comparison, I have implemented the zipping algorithm for a very similar, standard problem, 20 Newsgroups (determine the topic of a posting to one of 20 newsgroups.) I compared it to one of the simplest, easiest to implement algorithms used by Computer Scientists, the Naive Bayes model [4, page 238]. The zipping algorithm was 17 times slower and produced more than 3 times as many errors. It was only marginally simpler to implement, requiring 50 lines of perl script, instead of 70 lines. Omissions of this nature in a journal of the quality of Physical Review Letters are surprising and disappointing. In particular, although the concept of entropy originally was a Physics concept, it has been used and studied by Computer Scientists since the dawn of our field, and the information theoretic sense used here is not a strong link to physics. I suggest that authors writing letters essentially unrelated to physics may avoid embarrassing mistakes by submitting to journals that focus on the actual area of research, and that the editors of Physical Review Letters may avoid inappropriate publications by directing authors to those journals. Furthermore, editors and authors should be aware that computer scientists and other may feel insulted when those outside the field ignore its existence [3].