Stempel - Algorithmic Stemmer for Polish Language

Introduction

A method for conflation of different inflected word forms is an important component of many Information Retrieval systems. It helps to improve the system's recall and can significantly reduce the index size. This is especially true for highly-inflectional languages like those from the Slavic language family (Czech, Slovak, Polish, Russian, Bulgarian, etc).

This page describes a software package consisting of high-quality stemming tables for Polish, and a universal algorithmic stemmer, which operates using these tables. The stemmer code is taken virtually unchanged from the Egothor project.

The software distribution includes stemmer tables prepared using an extensive corpus of Polish language (see details below).

This work is available under Apache-style Open Source license - the stemmer code is covered by Egothor License, the tables and other additions are covered by Apache License 2.0. Both licenses allow to use the code in Open Source as well as commercial (closed source) projects.

Terminology

A short explanation is in order about the terminology used in this text.

In the following sections I make a distinction between stem and lemma.

Lemma is a base grammatical form (dictionary form, headword) of a word. Lemma is an existing, grammatically correct word in some human language.

Stem on the other hand is just a unique token, not necessarily making any sense in any human language, but which can serve as a unique label instead of lemma for the same set of inflected forms. Quite often stem is referred to as a "root" of the word - which is incorrect and misleading (stems sometimes have very little to do with the linguistic root of a word, i.e. a pattern found in a word which is common to all inflected forms or within a family of languages).

For an IR system stems are usually sufficient, for a morphological analysis system obviously lemmas are a must. In practice, various stemmers produce a mix of stems and lemmas, as is the case with the stemmer described here. Additionally, for some languages, which use suffix-based inflection rules many stemmers based on suffix-stripping will produce a large percentage of stems equivalent to lemmas. This is however not the case for languages with complex, irregular inflection rules (such as Slavic languages) - here simplistic suffix-stripping stemmers produce very poor results.

Background

Lemmatization is a process of finding the base, non-inflected form of a word. The result of lemmatization is a correct existing word, often in nominative case for nouns and infinitive form for verbs. A given inflected form may correspond to several lemmas (e.g. "found" -> find, found) - the correct choice depends on the context.

Stemming is concerned mostly with finding a unique "root" of a word, which not necessarily results in any existing word or lemma. The quality of stemming is measured by the rate of collisions (overstemming - which causes words with different lemmas to be incorrectly conflated into one "root"), and the rate of superfluous word "roots" (understemming - which assigns several "roots" to words with the same lemma).

Both stemmer and lemmatizer can be implemented in various ways. The two most common approaches are:

There are many existing and well-known implementations of stemmers for English (Porter, Lovins, Krovetz) and other European languages (Snowball). There are also good quality commercial lemmatizers for Polish. However, there is only one freely available Polish stemmer, implemented by Dawid Weiss, based on the "ispell" dictionary and Jan Daciuk's FSA package. That stemmer is dictionary-based. This means that even though it can achieve perfect accuracy for previously known word forms found in its dictionary, it completely fails in case of all other word forms. This deficiency is somewhat mitigated by the comprehensive dictionary distributed with this stemmer (so there is a high probability that most of the words in the input text will be found in the dictionary), however the problem still remains (please see the page above for more detailed description).

The implementation described here uses an algorithmic method. This method and particular algorithm implementation are described in detail in [1][2]. The main advantage of algorithmic stemmers is their ability to process previously unseen word forms with high accuracy. This particular algorithm uses a set of transformation rules (patch commands), which describe how a word with a given pattern should be transformed to its stem. These rules are first learned from a training corpus. They don't cover all possible cases, so there is always some loss of precision/recall (which means that even the words from the training corpus are sometimes incorrectly stemmed).

Algorithm and implementation

The algorithm and its Java implementation is described in detail in the publications cited below. Here's just a short excerpt from [2]:

"The aim is separation of the stemmer execution code from the data structures [...]. In other words, a static algorithm configurable by data must be developed. The word transformations that happen in the stemmer must be then encoded to the data tables.

The tacit input of our method is a sample set (a so-called dictionary) of words (as keys) and their stems. Each record can be equivalently stored as a key and the record of key's transformation to its respective stem. The transformation record is termed a patch command (P-command). It must be ensured that P-commands are universal, and that P-commands can transform any word to its stem. Our solution[6,8] is based on the Levenstein metric [10], which produces P-command as the minimum cost path in a directed graph.

One can imagine the P-command as an algorithm for an operator (editor) that rewrites a string to another string. The operator can use these instructions (PP-command's): removal - deletes a sequence of characters starting at the current cursor position and moves the cursor to the next character. The length of this sequence is the parameter; insertion - inserts a character ch, without moving the cursor. The character ch is a parameter; substitution  - rewrites a character at the current cursor position to the character ch and moves the cursor to the next character. The character ch is a parameter; no operation (NOOP) - skip a sequence of characters starting at the current cursor position. The length of this sequence is the parameter.

The P-commands are applied from the end of a word (right to left). This assumption can reduce the set of P-command's, because the last NOOP, moving the cursor to the end of a string without any changes, need not be stored."

Data structure used to keep the dictionary (words and their P-commands) is a trie. Several optimization steps are applied in turn to reduce and optimize the initial trie, by eliminating useless information and shortening the paths in the trie.

Finally, in order to obtain a stem from the input word, the word is passed once through a matching path in the trie (applying at each node the P-commands stored there). The result is a word stem.

Corpus

(to be completed...)

The following Polish corpora have been used:

This step was the most time-consuming - and it would probably be even more tedious and difficult if not for the help of Python. The source texts had to be brought to a common encoding (UTF-8) - some of them used quite ancient encodings like Mazovia or DHN - and then scripts were written to collect all lemmas and inflected forms from the source texts. In cases when the source text was not tagged, I used the SAM analyzer to produce lemmas. In cases of ambiguous lemmatization I decided to put references to inflected forms from all base forms.

All grammatical categories were allowed to appear in the corpus, i.e. nouns, verbs, adjectives, numerals, and pronouns. The resulting corpus consisted of roughly 87,000+ inflection sets, i.e. each set consisted of one base form (lemma) and many inflected forms. However, because of the nature of the training method I restricted these sets to include only those where there were at least 4 inflected forms. Sets with 3 or less inflected forms were removed, so that the final corpus consisted of ~69,000 unique sets, which in turn contained ~1.5 mln inflected forms.

Testing

I tested the stemmer tables produced using the implementation described above. The following sections give some details about the testing setup.

Testing procedure

The testing procedure was as follows:

Test results

The following table summarizes test results for varying sizes of training samples. The meaning of the table columns is described below:

Training sets Testing forms Stem OK Lemma OK Missing Stem Bad Lemma Bad Table size [B]
100 1022985 842209 593632 172711 22331 256642 28438
200 1022985 862789 646488 153288 16306 223209 48660
500 1022985 885786 685009 130772 14856 207204 108798
700 1022985 909031 704609 107084 15442 211292 139291
1000 1022985 926079 725720 90117 14941 207148 183677
2000 1022985 942886 746641 73429 14903 202915 313516
5000 1022985 954721 759930 61476 14817 201579 640969
7000 1022985 956165 764033 60364 14620 198588 839347
10000 1022985 965427 775507 50797 14662 196681 1144537
12000 1022985 967664 782143 48722 14284 192120 1313508
15000 1022985 973188 788867 43247 14349 190871 1567902
17000 1022985 974203 791804 42319 14333 188862 1733957
20000 1022985 976234 791554 40058 14601 191373 1977615

I also measured the time to produce a stem (which involves traversing a trie, retrieving a patch command and applying the patch command to the input string). On a machine running Windows XP (Pentium 4, 1.7 GHz, JDK 1.4.2_03 HotSpot), for tables ranging in size from 1,000 to 20,000 cells, the time to produce a single stem varies between 5-10 microseconds.

This means that the stemmer can process up to 200,000 words per second, an outstanding result when compared to other stemmers (Morfeusz - ~2,000 w/s, FormAN (MS Word analyzer) - ~1,000 w/s).

The package contains a class org.getopt.stempel.Benchmark, which you can use to produce reports like the one below:

--------- Stemmer benchmark report: -----------
Stemmer table:  /res/tables/stemmer_2000.out
Input file:     ../test3.txt
Number of runs: 3

 RUN NUMBER:            1       2       3
 Total input words      1378176 1378176 1378176
 Missed output words    112     112     112
 Time elapsed [ms]      6989    6940    6640
 Hit rate percent       99.99%  99.99%  99.99%
 Miss rate percent      00.01%  00.01%  00.01%
 Words per second       197192  198584  207557
 Time per word [us]     5.07    5.04    4.82

Summary

The results of these tests are very encouraging. It seems that using the training corpus and the stemming algorithm described above results in a high-quality stemmer useful for most applications. Moreover, it can also be used as a better than average lemmatizer.

Both the author of the implementation (Leo Galambos, <leo.galambos AT egothor DOT org>) and the author of this compilation (Andrzej Bialecki <ab AT getopt DOT org>) would appreciate any feedback and suggestions for further improvements.

Bibliography

  1. Galambos, L.: Multilingual Stemmer in Web Environment, PhD Thesis, Faculty of Mathematics and Physics, Charles University in Prague, in press.
  2. Galambos, L.: Semi-automatic Stemmer Evaluation. International Intelligent Information Processing and Web Mining Conference, 2004, Zakopane, Poland.
  3. Galambos, L.: Lemmatizer for Document Information Retrieval Systems in JAVA. <http://www.informatik.uni-trier.de/%7Eley/db/conf/sofsem/sofsem2001.html#Galambos01> SOFSEM 2001, Piestany, Slovakia.