Advanced NLP Interview Questions, issue 25, Dec 30, 2025
The Back-Translation Direction Trap
Senior NLP Engineer interview at Google DeepMind, and the interviewer asks:
βWe need to improve our ππ’π±π’π―π¦π΄π¦-π΅π°-ππ―π¨ππͺπ΄π© translation model. We have 10k parallel pairs and 1 billion lines of monolingual English text. To use ππππ€-ππ«ππ§π¬π₯πππ’π¨π§ effectively, which direction do we generate data, and exactly how do we pair it for training?β
Donβt say: βWe translate the English data into Japanese to check if the model is consistent.β
Why generating synthetic sources (not targets) is the only way to preserve decoder fluency in production NMT systems.
The full answer, with the mechanism and the arithmetic, is for paid subscribers on Substack.