When you create and name your Decks, you will want to make sure to organize them into the most convenient format for your study purposes, or the purposes of your students.
It usually makes more sense to separate decks topically (e.g. Cell Division, Organelles, etc.) rather than by question type (Vocab, Images, Practice Questions, etc.), but that's up to you.
The right Deck size is the size that fully meets the learning objectives of the target user. No more, no less. Some Classes can be broken down into anywhere between three and 40 Decks, with anywhere between 15 and 400 cards in each. The sweet spot is generally between 20 and 200 cards per deck.
Decks that get too small -- fewer than 20 cards -- do not leverage Brainscape's CBR study algorithm optimally. Very quickly, the system may begin repeating the same few cards over and over again, which does not optimize learning.
Decks that get too large -- greater than 200-300 cards -- may become frustrating for some learners who want to feel a greater sense of incremental "completion".
While you usually don't need to separate Decks into artificially small chunks, that may occasionally be helpful if you have a TON of cards in one particular sub-topic, and you want to separate it into smaller Decks so that users have more "achievable" short-term milestones.
e.g. If your "Cell Division" Deck has over 400 flashcards, you might want to separate it into "Cell Division: Set 1" and "Cell Division: Set 2", or some other breakdown that suits your purposes.
At the end of the day, your Class and Deck organization is up to you, so do whatever best suits your needs!
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