Published on October 4, 2018. Last Updated on May 6, 2023.

## Spoiler alert: game design uses math, whether you’re good at it or not.

Designing a game means one is working with probabilities, and changing them to fit your needs is part of the design process. You’ll also want to know how often (or rarely) a certain thing may happen, Designing a game means thinking about permutations, combinations, and might even involve a hypergeometric calculator. (Yes, this is a real thing. It’s pretty cool).

## The… hypergeometric calculator?!?

Yep, that’s what it’s called. It’s a really fancy way of saying ‘the probability of something happening’. If there are five bad cards in a deck of 25 cards, what are the chances of drawing, say, 1 bad card when you draw 7 cards? I could play hundreds of rounds and count how often this oddity happened, but math saves plenty of time and effort.

Start by going to http://stattrek.com/online-calculator/hypergeometric.aspx:

Stat Trek asks for four things here, so let’s plug-in some numbers from my previous example (From my previous example (“If there are 5 bad cards in a deck of 25 cards, what are the chances of drawing, say, 1 bad card when you draw 7 cards?”)

**Population size**is the overall universe or deck of cards to choose from. Here, it’s 25.**Number of Successes in Population**is the number of bad cards, or the cards you’re trying to see the probability. Here, it’s 5.**Sample Size**is the number of cards you’re drawing from that universe or deck of cards. Here, it’s 7.**Number of successes in sample (x)**is the number of bad cards, or the cards you’re trying to see the probabiliity*in that sample size.*Here, it’s 1.

Type all those into the calculator, and….

*OK, so the grey boxes got filled in. How does this help me?* The grey boxes show the probabilities of that thing happening from 0 to 1, or 0% to 100%. Multiply each number by 100 to get their percentage.

That first box, P(X = 1)? That’s the one you want. Given this set up, the probability that you’ll draw exactly 1 bad card when you draw 7 cards is 0.403162055, or 40.3%. The other numbers are *cumulative probabilities*, which consider all the other options:

- Cumulative Probability: P(X < 1): 0.161264822 — the chance you’ll draw
**less than 1**bad card (in other words, 0) is 16.1%. - Cumulative Probability: P(X
__<__1) : 0.564426877 — the chance you’ll draw**1 or fewer**bad cards (in other words, 0 or 1) is 56.4%. - Cumulative Probability: P(X > 1) : 0.435573123 — the chance you’ll draw
**more than 1**bad card (in other words, 2 to 5) is 43.6%. - Cumulative Probability: P(X
__>__1) : 0.838735178 — the chance you’ll draw**1 or more**bad cards (in other words, 1 to 5) is 83.9%.

I’ll note that this last one is actually the most important — you have to always consider the chance they’ll draw *more* than one bad card.

## Results – yay! Now what?

Well, now, the most obvious is ‘what sort of probabilities am I looking for?’ Drawing 1 or more bad cards in this setup looks pretty common, so does that work for your gameplay? You might say ‘*actually, I want this to be a 60% probability to draw 1 or more bad cards instead…’ — *so let’s play with the numbers here.

- To
**decrease**the chance of something happening,**increase**the population size and/or**decrease**the number of successes. - To
**increase**the chance of something happening,**decrease**the population size and/or**increase**the number of successes.

Let’s play with the numbers:

Success! We’ve gone from a deck of 25 to a deck of 32 cards, and now we’re drawing 5 cards instead of 7. The probability of drawing one or more bad cards is now 0.599108136, or 59.91%.

It might go without saying, but you’ll have to balance the desire for a specific probability with the rest of the story. Trying to engineer the exact balance for every type of card in your game is likely going to be tough work, though, especially to keep to the structures and phases of the game you have in mind.

Want some help working out the math in your game?

Let's chat one-on-one## Over to you

What’s been your biggest struggle with math in a game?