We've been teaching ESL and adult literacy for many years and seen various schemes for measuring readability wax and wane in popularity. Additionally, PageTurners are sold internationally and different countries do things differently.
We didn't want to tie ourselves to any particular scheme, so when we were working out our levels we made it really simple — the number of unique words in a text, and the length of the text.
So...
Level 1 (yellow covers)
— about 100 words long
— up to 60 unique words
Level 2 (red covers)
— about 200 words long
— up to 150 unique words
Level 3 (blue covers)
— around 300 words in total
— about 200 unique words
Level 4 (green covers)
— 400 - 600 words long
— about 300 unique words
But it's a little more than that. Because we're experienced teachers we also go by instinct. We'll replace a difficult word with a simple one when we can. We hope the pictures will add to the ease of reading the text. We read the stories aloud and check that the sentences don't get too long or involved, especially at the lower levels. We usually trial them with adult learners as well.
First and foremost we want the books to be fun to read but we also want them to be useful and lead to good discussions. We're always happy to consider suggestions and answer questions.
