Our difficulty ratings are powered by our community. Any user can grade their completed books by comparing two books at a time - either choosing one book to be more difficult or deeming both books to be about the same difficulty.
For example, if you've read both よつばと！ and ふらいんぐういっち, you'll be prompted to compare them as shown below:
These comparisons are then put into an Elo based rating system which produces the difficulty levels you see on the site.
You can view the raw comparisons for each book in the gradings section on its associated page, such as よつばと! 1.
There is no formal criteria. Rather, each user grades the difficulty as they see fit - some users will deem strange vocabulary as more difficult than complex grammar or vice versa. This hopefully averages out to a general language learner's perspective.
While we strongly recommend that you think of the levels as relative levels (a level 15 book is harder than a level 5 book), we do showcase a very approximate mapping to the JLPT proficiency levels. This is just a best guess - don't be afraid to try content deemed 'above' or 'below' your JLPT level.
Like we said above, this mapping to JLPT is only an estimation - our system is focused on perception of difficulty, not JLPT guidelines. User perception of difficulty may factor in many things totally unrelated to JLPT, like text density, writing style, book length, or slang. The JLPT estimation is just a rough guide and hopefully helpful to our users.
The JLPT estimation is intended to denote content that's appropriate for that skill level. For example, the JLPT N3 content is targeted towards learners who have either attained the JLPT N3 certificate or who are studying for the JLPT N3 exam and want challenging reading material.
Each book is created with an initial estimated level (chosen by our team or the user who requested the resource). Until the book has been graded a few times, the difficulty level will fluctate significantly. These temporary grades are marked by '??'.
You will notice that most series have the same difficulty rating across all volumes - this is by design. In order to focus on more meaningful comparisons, we generally do not allow difficulty comparisons between volumes on the same series. However, if a series does range significantly in difficulty, please let us know and we can treat that series on a per volume basis. There are a few exceptional series where this is the case (Harry Potter, Graded Reader series... etc).
Another item to note: Even on series where the difficulty rating is the same across volumes, we still always ask you to compare a particular volume in your difficulty comparisons. This allows us to change the series to be handled on a per volume basis at a later date
In the future, we will improve this series handling so that you can compare volumes on any series if you want to, but we have other priorities right now!
When you compare two items which still have uncertain difficulty ratings (??), we can't immediately process the grading. Rather, we keep it stored until one of the items becomes a confident rating.
These gradings are very important, especially in 'Beta' languages where there are very few certain difficulty ratings.
If we have lots of comparisons between uncertain items, sometimes it only takes the change in one item's certainty to impact the certainty of a whole group of items!
Currently, we only auto generate 6 gradings per book for you. However, if there's a particular book which you would like to grade more, go to book page and see if there's an '+ grade extra' button near the grading section header. This will allow you to do more gradings on a particular book.
Unfortunately, we can't incorporate these grades into our ratings currently, as it would give particular users too much influence, but eventually we will incorporate them, so we appreciate them! They will also be displayed on the book page as well, so they still are very useful.
Some categories of books (currently only textbooks) are excluded from user gradings, with the level simply specified by our team. It's nonsensical to compare a textbook vs a manga in terms of difficulty. We do want to incorporate user gradings of textbooks at some point, but currently we do not.
Without our community grading books, this wouldn't be possible. Thank you so much! Additionally, if you have any large disagreements with a particular rating, please contact us!