When editing an exercise in the authoring environment, under the "Info" tab, there is a tab "Rating".

The idea for the exercise rating came from the system of Elo rating in chess. The Elo rating system measures the relative strength of a player compared to other players. On our platform, the student is one player, and the exercise is the opposing player.

The scale goes from 0 to 1000, and it shows how likely the exercise is to "win" over the student. A higher rating for exercises means that the exercise has a higher chance of winning (that the student does not solve the exercise).
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We aren't actively working on this functionality, so the ratings might not be precise.