Dataset-Length Bias

Neural Machine Translation (NMT) is known to suffer from a beam-search problem: after a certain point, increasing beam size causes an overall drop in translation quality. This effect is especially in long sentences. A factor that strongly contributes to the quality degradation with large beams is dataset-length bias which means that NMT datasets are strongly biased towards short sentences.

To mitigate this issue, some researchers from Yandex in 2021 proposed a new data augmentation technique called “Multi-Sentence Resampling” or MSR for short in their paper: Multi-Sentence Resampling: A Simple Approach to Alleviate Dataset Length Bias and Beam-Search Degradation. This technique extends the training examples by concatenating several sentences from the original dataset to make a long training example. The official implementation of this technique can be found on the Yandex official GitHub Repository: msr.


MSR stands for “Multi-Sentence Resampling” which augments a dataset such that each training example consists of 1 to N sentences randomly chosen from a dataset and concatenated one after another preserving the order of sentence. The following is the full algorithm:

As shown in the following example, we have a dataset of just three short sentences (on the left). Using MSR with N=3 will create a new dataset where each sentence is either 1 or 2 or 3 sentences long. MSR concatenates sentences together as shown on the right table which increases the average length of the dataset.


  • MSR performed the augmentation process on both the source-side and the target-side of the data.

  • Knowing the average length of examples in a dataset $L$, the average length of examples in the new dataset can be approximately calculated as:

\[avg\_ new\_ length \cong \sum_{n = 1}^{N}\frac{\text{L.n}}{N} = L.\frac{N + 1}{2}\]

The following figure illustrates how the train examples length distribution changes in IWSLT17 Fr-En dataset for N from 2 to 5. With growing N distributions become more flatten for lengths presented in the test set.