Data filtering

When you submit documents to be used for training, the documents undergo a series of processing and filtering steps. These steps are explained here. The knowledge of the filtering may help you understand the sentence count displayed in Custom Translator and the steps you may take yourself to prepare the documents for training with Custom Translator.

Sentence alignment

If your document isn't in XLIFF, TMX, or ALIGN format, Custom Translator aligns the sentences of your source and target documents to each other, sentence by sentence. Custom Translator doesn't perform document alignment – it follows your naming of the documents to find the matching document of the other language. Within the document, Custom Translator tries to find the corresponding sentence in the other language. It uses document markup like embedded HTML tags to help with the alignment.

If you see a large discrepancy between the number of sentences in the source and target documents, your documents may not be parallel. The document pairs with a large difference (>10%) of sentences on each side warrant a second look to make sure they're indeed parallel. Custom Translator shows a warning next to the document if the sentence count differs suspiciously.


Custom Translator removes the sentences that are present in test and tuning documents from training data. The removal happens dynamically inside of the training run, not in the data processing step. Custom Translator reports the sentence count to you in the project overview before such removal.

Length filter

  • Remove sentences with only one word on either side.
  • Remove sentences with more than 100 words on either side.  Chinese, Japanese, Korean are exempt.
  • Remove sentences with fewer than three characters. Chinese, Japanese, Korean are exempt.
  • Remove sentences with more than 2000 characters for Chinese, Japanese, Korean.
  • Remove sentences with less than 1% alpha characters.
  • Remove dictionary entries containing more than 50 words.

White space

  • Replace any sequence of white-space characters including tabs and CR/LF sequences with a single space character.
  • Remove leading or trailing space in the sentence

Sentence end punctuation

Replace multiple sentence end punctuation characters with a single instance.

Japanese character normalization

Convert full width letters and digits to half-width characters.

Unescaped XML tags

Filtering transforms unescaped tags into escaped tags:

  • < becomes <
  • > becomes >
  • & becomes &

Invalid characters

Custom Translator removes sentences that contain Unicode character U+FFFD. The character U+FFFD indicates a failed encoding conversion.

Next steps