A thorough human evaluation performed on over 3,000 sentence pairs showed that the accuracy of automatic alignment between the sentences from two neighboring Newsela levels of simplification is between 83% (for levels 3 and 4) and 98% (for levels 0 and 1), while the alignment method aligns sentences from the hardest and the easiest levels (0 and 4), with only 58% accuracy (Štajner et al., 2017).
we aligned and used only the neighboring Newsela levels (0-1, 1-2, 2-3, and 3-4) as training data.