Draw Luck in Card Games, Part 2


A few weeks ago I talked about draw luck in card games. I thought I’d go a little further today with the actual math behind some core concepts when you play a card game where you build your own deck to use. The same idea works for computing probabilities in poker, so you don’t need to get too hung up on the particulars here.

I’m going to use Magic: The Gathering (MTG) as an example. Here are the relevant idea axioms we will use:

1. Your deck will consist of 60 cards.
2. You start by drawing 7 cards.
3. Each turn you draw 1 card.
4. Each card has a “cost” to play it (called mana).
5. Optimal strategy is to play a cost 1 card on turn 1, a cost 2 card on turn 2, and so on. This is called “playing on curve.”

You don’t have to know anything about MTG now that you have these axioms (in fact, writing them this way allows you to convert everything to Hearthstone, or your card game of choice). Of course, every single one of those axioms can be affected by play, so this is a vast oversimplification. But it gives a good reference point if you’ve never seen anything like this type of analysis before. Let’s build up the theory little by little.

First, what is the probability of being able to play a 1-cost card on turn 1 if you put, say, 10 of these in your deck? We’ll simplify axiom 2 to get started. Suppose you only draw one card to start. Basically, by definition of probability, you have a 10/60, or 16.67% chance of drawing it. Now if you draw 2 cards, it already gets a little bit trickier. Exercise: Try to work it out to see why (hint: the first card could be 1-cost OR the second OR both).

Let’s reframe the question. What’s the probability of NOT being able to play a card turn 1 if you draw 2 cards? You would have to draw a non-1-cost AND another non-1-cost. The first card you pick up has a 50/60 chance of this happening. Now the deck only has 59 cards left, and 49 of those are non-1-cost. So the probability of not being able to play turn 1 is {\frac{50}{60}\cdot\frac{49}{59}}, or about a 69% chance.

To convert this back, we get that the probability of being able to play the 1-cost card on turn 1 (if start with 2 cards) is {\displaystyle 1- \frac{50\cdot 49}{60\cdot 59}}, or about a 31% chance.

Axiom 2 says that in the actual game we start by drawing 7 cards. The pattern above continues in this way, so if we put {k} 1-cost cards in our deck, the probability of being able to play one of these on turn 1 is:

{\displaystyle 1 - \frac{(60-k)\cdot (60-k-1)\cdots (60-k-7)}{60\cdot 59\cdots (60-7)} = 1 - \frac{{60-k \choose 7}}{{60 \choose 7}}}.

To calculate the probability of hitting a 2-cost card on turn 2, we just change the 7 to an 8, since we’ll be getting 8 cards by axiom 3. The {k} becomes however many 2-cost cards we have.

Here’s a nice little question: Is it possible to make a deck where we have a greater than 50% chance of playing on curve every turn for the first 6 turns? We just compute the {k} above that makes each probability greater than {0.5}. This requires putting the following amount of cards in your deck:

6 1-cost
5 2-cost
5 3-cost
4 4-cost
4 5-cost
3 6-cost

Even assuming you put 24 lands in your deck, this still gives you tons of extra cards. Let’s push this a little further. Can you make a deck that has a better than 70% chance of playing on curve every turn? Yes!

9 1-cost
8 2-cost
7 3-cost
7 4-cost
6 5-cost
6 6-cost

Warning: This mana curve would never be used by any sort of competetive deck. This is a thought experiment with tons of simplifying assumptions. The curve for your deck is going to depend on a huge number of things. Most decks will probably value playing on curve in the 2,3,4 range way more than other turns. If you have an aggressive deck, you might value the early game. If you play a control deck, you might value the later game.

Also, the longer the game goes, the less cards you probably need in the high cost range to get those probabilities up, because there will be ways to hunt through your deck to increase the chance of finding them. Even more, all of these estimates are conservative, because MTG allows you to mulligan a bad starting hand. This means many worst-case scenarios get thrown out, giving you an even better chance at playing on curve.

This brings us back to the point being made in the previous post. Sometimes what feels like “bad luck” could be poor deck construction. This is an aspect you have full control over, and if you keep feeling like you aren’t able to play a card, you might want to work these probabilities out to make a conscious choice about how likely you are to draw certain cards at certain points of the game.

Once you know the probabilities, you can make more informed strategic decisions. This is exactly how professional poker is played.

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