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As-fan is an internal term used by Magic: the Gathering designers to talk about the likelihood of a card of group of cards appearing in a pack. Mark Rosewater’s has written an article discussing this in detail and I strongly recommend reading it.
The term itself is not important (I think of it as ‘as-found’) but what is interesting is what the terms means, and its implications of game designers and card collectors.
If a card type or mechanic, or even theme is important to your game, your players need to be coming across that theme relatively often during the game, or it’s unlikely to really affect them. This is really what as-fan is talking about – frequency.
Something that you as a designer view as fundamental to the player experience you are trying to create needs to be in front of the players. Magic’s designers understand this, and use as-fan as they build new sets to ensure that certain themes or mechanics are at the level they need to be to connect with players.
Calculating as-fan requires a certain amount of maths.
From the article: “Now let’s say you add 10 rares. Your as-fan goes up to 1.540. Here’s why. If you have 10 rares, that means you have a 10 in 53 (or 0.189) chance of getting one in your rare slot, as a large set has 53 rares. Here’s where rares get tricky. Because you only get a rare in seven out of eight packs, on average, you count the rare slot as 7/8 (or 0.875) of a slot. This means you multiply 0.189 by 0.875 to get 0.165. When you add that to 1.375 (the as-fan with 10 commons and 10 uncommons) you get 1.540.”
Scary to the likes of me, but also quite important.
As-fan is worth calculating in a number of game design endeavours:
I’m sure you can think of many more examples. In game design, hard maths can go alongside gut feeling to give designers the input they need.