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Directional Skip-Gram: Explicitly Distinguishing Left and Right Context for Word Embeddings

Paper · 2018 · Yan Song, Shuming Shi, Jing Li et al.

Summary

The standard skip-gram (SG) model learns word vectors from co-occurrence alone and does not register whether a neighbour appears before or after the target word. Earlier order-aware extensions, such as the structured skip-gram (SSG), keep a separate output vector for each relative position. This inflates memory and training time and makes them vulnerable to sparse data. Song et al. (Tencent AI Lab) propose directional skip-gram (DSG). It gives every word one extra "direction" vector, which is trained with a sigmoid objective to tell whether a context word lies to the left or to the right. This direction score is added to the usual SG prediction. The extra cost is one operation per word, so training speed remains close to SG.

Embeddings were trained on an English Wikipedia dump of roughly 2 billion tokens. They were evaluated on word similarity and on POS tagging, the latter with a BiLSTM-CRF. DSG outperformed SG, SSG and a simplified SSG (SSSG) in both settings, and it held up especially well on a tiny training corpus. The result shows that a cheap, binary notion of word order can improve embeddings without extra resources.

Key points

  • Cost: DSG and SSSG both need 3|V|d parameters, compared with 2|V|d for SG and (2c+1)|V|d for SSG. Operations scale as 2cC(n+2)o for DSG versus 4c²C(n+1)o for SSG, so DSG's speed gap to SG narrows as the context window widens.
  • Training setup: all models shared the same hyperparameters, namely 200 dimensions, window 5, 5 negative samples, 5 iterations and a starting learning rate of 0.025.
  • Word similarity, full Wiki corpus: DSG was the best of the SG-family models, with Spearman ρ×100 of 73.76 on MEN-3k, 36.10 on SimLex-999 and 72.60 on WS-353. CWin scored higher only on MEN-3k (74.28).
  • Word similarity, 0.1% Wiki sample: this sample simulated a cold-start setting, and DSG was best on all three datasets (63.18, 27.51 and 66.71). SSG and CWin degraded markedly on the same sample.
  • POS tagging, test accuracy: on WSJ (Penn Treebank) DSG reached 97.37, ahead of SSG at 97.31 and SG at 97.12. On the ARK tweets it reached 90.43, ahead of SSG at 90.15 and SG at 88.77; the ARK embeddings came from a 100-million-tweet TWT corpus.

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

Cite

Cite as Yan Song, Shuming Shi, Jing Li and Haisong Zhang. 2018. Directional Skip-Gram: Explicitly Distinguishing Left and Right Context for Word Embeddings. Proceedings of the 2018 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies, Volume 2 (Short Papers).

The paper itself, not this page. BibTeX from the library's record of the work. DOI 10.18653/v1/n18-2028

@inproceedings{song2018directional,
  title = {{Directional Skip-Gram: Explicitly Distinguishing Left and Right Context for Word Embeddings}},
  author = {Yan Song, Shuming Shi, Jing Li and Haisong Zhang},
  booktitle = {Proceedings of the 2018 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies, Volume 2 (Short Papers)},
  year = {2018},
  doi = {10.18653/v1/n18-2028},
  url = {https://doi.org/10.18653/v1/n18-2028}
}

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