---
title: "Improving Accuracy in word class tagging through the"
description: "The paper tests whether different data-driven learners trained on the same NLP task make errors that… 48 references, cited by 19 papers here."
authors: ["Hans Van Halteren", "Jakub Zavrel", "Walter Daelemans"]
year: 2001
venue: "Computational Linguistics"
source: "https://aclanthology.org/J01-2002/"
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summary_licence: "Elamur Cosmic License"
canonical: "https://research.elamur.ai/free/en/papers/halteren-2001-improving-accuracy-word-class-tagging/"
---
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# Improving Accuracy in word class tagging through the Combination of Machine Learning Systems

**Authors:** Hans Van Halteren, [Jakub Zavrel](https://md.research.elamur.ai/free/en/authors/jakub-zavrel/) and [Walter Daelemans](https://md.research.elamur.ai/free/en/authors/walter-daelemans/) · **Year:** 2001 · **Venue:** [Computational Linguistics](https://md.research.elamur.ai/free/en/venues/cl/) ([Computational Linguistics 2001](https://md.research.elamur.ai/free/en/venues/cl/2001/)) · **Publisher:** [The MIT Press](https://md.research.elamur.ai/free/en/publishers/the-mit-press/) · **Licence:** [CC BY-NC-SA 3.0 (ACL Anthology, before 2016)](https://creativecommons.org/licenses/by-nc-sa/3.0/)

**Links:** [Publisher's page](https://aclanthology.org/J01-2002/) · [PDF](https://aclanthology.org/J01-2002.pdf) · [OpenAlex](https://openalex.org/W2154210598)

**Citations:** 185 citations (OpenAlex, 2026-10-14) · [19](https://md.research.elamur.ai/free/en/papers/halteren-2001-improving-accuracy-word-class-tagging/cited-by/) in our library, of the 149,439 papers whose bibliographies we read

## Summary
The paper tests whether different data-driven learners trained on the same NLP task make errors that differ enough for a combined system to beat the best single learner. The task is morphosyntactic word class (POS) tagging. Data-driven taggers already reach about 96–97% accuracy on this task but still do poorly on certain ambiguous words. The authors train four off-the-shelf tagger generators on identical data from three tagged corpora, one of which is LOB. The generators are a hidden Markov model, a memory-based tagger, transformation rules and maximum entropy. Their outputs are merged in two ways. Voting schemes, plain or weighted by precision and recall, produce a "gang" effect. Stacked second-level classifiers, which learn from patterns of agreement and disagreement, produce an "arbiter" effect. Cross-validation gives these second-stage learners far more training material than the authors' 1998 study had. Every combined tagger is more accurate than its strongest component, and on LOB the error rate falls by up to 24.3%. The study shows that combining heterogeneous learners can improve accuracy without new representations or extra annotated data.

## Key points
- Four component tagger generators are used: HMM, memory-based (related tools: MBT, TiMBL), transformation-based rules and maximum entropy. All are trained on the same corpus material.
- The voting variants are Majority, TotPrecision, TagPrecision and Precision-Recall, with ties broken at random.
- The stacking combiners use memory-based learning, maximum entropy and decision trees as second-stage learners. The paper also introduces a new method based on grouped voting.
- The best result is a 24.3% error-rate reduction on the LOB corpus. Earlier work by the same authors (1998) had reported a 19.1% reduction for its best combiner.
- Cross-validation was used to enlarge the second-level training data. It addresses the earlier failure to clearly confirm that arbiter-style combiners outperform voting.

*Our summary of the paper, in our own words — not its text. The work belongs to its authors; read it at the links above.*

## Institutions

- [University of Antwerp](https://md.research.elamur.ai/free/en/universities/university-of-antwerp/) — University, Antwerp, Belgium · 17 works · 3,490 citations
- [Radboud University Nijmegen](https://md.research.elamur.ai/free/en/universities/radboud-university-nijmegen/) — University, Nijmegen, Netherlands · 20 works · 1,688 citations

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## Cite

Cite as: Hans Van Halteren, Jakub Zavrel and Walter Daelemans. 2001. Improving Accuracy in word class tagging through the Combination of Machine Learning Systems. Computational Linguistics.

The paper itself, not this page. BibTeX from the library's record of the work.

```bibtex
@article{halteren2001improving,
  title = {{Improving Accuracy in word class tagging through the Combination of Machine Learning Systems}},
  author = {Hans {Van Halteren}, Jakub Zavrel and Walter Daelemans},
  journal = {Computational Linguistics},
  year = {2001},
  doi = {10.1162/089120101750300508},
  url = {https://doi.org/10.1162/089120101750300508}
}
```

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*Updated 31 October 2026*