Boosting Zero-shot Cross-lingual Retrieval by Training on Artificially Code-Switched Data
This addresses a key bottleneck in cross-lingual information retrieval for users and systems dealing with multiple languages, offering a cheap and effective method to improve performance, though it is incremental as it builds on existing zero-shot transfer approaches.
The paper tackles the problem of diminished effectiveness in zero-shot cross-lingual information retrieval when queries and documents are in different languages, by proposing to train ranking models on artificially code-switched data generated using bilingual lexicons, resulting in substantial gains of 5.1 MRR@10 in cross-lingual IR and 3.9 MRR@10 in multilingual IR.
Transferring information retrieval (IR) models from a high-resource language (typically English) to other languages in a zero-shot fashion has become a widely adopted approach. In this work, we show that the effectiveness of zero-shot rankers diminishes when queries and documents are present in different languages. Motivated by this, we propose to train ranking models on artificially code-switched data instead, which we generate by utilizing bilingual lexicons. To this end, we experiment with lexicons induced from (1) cross-lingual word embeddings and (2) parallel Wikipedia page titles. We use the mMARCO dataset to extensively evaluate reranking models on 36 language pairs spanning Monolingual IR (MoIR), Cross-lingual IR (CLIR), and Multilingual IR (MLIR). Our results show that code-switching can yield consistent and substantial gains of 5.1 MRR@10 in CLIR and 3.9 MRR@10 in MLIR, while maintaining stable performance in MoIR. Encouragingly, the gains are especially pronounced for distant languages (up to 2x absolute gain). We further show that our approach is robust towards the ratio of code-switched tokens and also extends to unseen languages. Our results demonstrate that training on code-switched data is a cheap and effective way of generalizing zero-shot rankers for cross-lingual and multilingual retrieval.