In which I discuss how random numbers can be a great tool for game designers

  • slash
  • 10/05/2013 12:39 AM

I’ve been wanting to write about "randomness" for a long time now. This article is an introduction to the idea of randomness in games, where I try to explain some of the basic principles of designing games with chance in mind and showcase some good examples of chance in game design. My goal is to encourage designers to think critically about how random numbers affect their games and gameplay, and possibly encourage some new ideas. Hope you like it!

We use randomness in our games all the time. What enemies appear, whether an attack hits or misses, how much damage our we deal: all determined by the roll of the dice. But why? What does random chance accomplish, and most importantly, how can we use it to make our games more fun?

This is the arbiter of your fate. This piece of plastic is responsible for your sorrow and your joy.

Designing For Luck

For the sake of this article, “random” refers to any element of a game that isn’t absolutely predetermined and can’t be absolutely controlled by the player. Game elements like the visuals, music or plot are rarely random (although they can be). Randomness is most useful when interacting with the choices a player must make while playing, and can be used to create stress, suspense, mystery and relief in your player.

Here are some examples of randomness in popular games:
  • The creature you encounter in tall grass or caves in Pokemon
  • The weather on a particular day in Harvest Moon
  • The damage an attack deals in Final Fantasy
  • The bullet spread on a gun in Call of Duty
  • The position of the mines in Minesweeper
  • The cards you’re dealt in a round of Blackjack

Note that even if a game element is technically random, if it doesn’t feel random to the player, it won’t create the subsequent suspense. For an element to feel random, it must recur often enough for the player to notice. Usually, this is because the game is short and is replayed often (such as a round of blackjack) or because the same situation occurs frequently in a single playthrough (such as a battle in Final Fantasy). If the situation only occurs once, the player will have no reason to believe it’s random at all - even if you tell them it is.

Randomness reduces skill…

It takes no mastery to roll a die. If a player rolls a d20, either the monster dies or it doesn’t. The attack hits, misses or crits and the player didn’t have any say in the matter. Their agency - the control the player has over their fate - is removed. This is especially contentious in certain genres that revolve around skill, such as multiplayer shooters, where your gun’s bullets fire in a psuedo-random direction determined by the spread of the gun.

Removing skill from a game isn’t always a bad thing, though. In a PVP-focused game like Team Fortress 2, random critical hits helps even the difference in skill, giving newbies the chance for a “lucky shot” at killing someone with more experience. While it’s often a source of minor irritation for experienced players, it makes the game more accessible to new players and encourages them to keep playing, so they too can become veterans and, inevitably, complain about critical hits.

But in the end, if your success or failure comes down to luck, you didn’t really have any say in matter… did you?

...except when it doesn’t.

As it turns out, you can design randomness to create skill-based decisions, and this is where the fun begins.

The Crossroads of Skill and Chance

Designing your game to make randomness fun for the player is tricky, but can lead to great results. Here, I'll try to categorize a few common techniques for using chance to create unique gameplay that relies on the player's knowledge and mastery of the game: influencing the dice, rejecting the dice, and parallel results.

Influencing the Dice

One common technique for unifying skill and randomness is giving the player the ability to influence the dice. Anything the player can do to manipulate the usually-random numbers falls under this category. A spell that increases evasion influences the dice - the player is much more likely to dodge attacks. Since the player has a choice to make (“Do I cast Evasion?”), the player retains agency, because they have at least some control over their fate. You can design your game with situations where a boosted evasion state is useful, and thus reward the player for cleverly manipulating the dice.

Examples of influencing the dice:
  • Buffs and debuffs that alter accuracy or critical hit chance
  • The ability to “reroll” the results of randomization
  • “Magic find” stats that increase the likelihood of finding more powerful items

Rejecting the Dice

With the right design, randomness can create another type of choice: to accept or reject the dice. This refers to any situation where the player can knowledgeably choose not to participate in a dice roll. For example, Final Fantasy 7’s Deathblow ability can either deal massive damage or miss completely, and has a pretty good chance of doing either. In some situations, using Deathblow is a great decision, such as when the massive damage will finish off that last enemy and save time waiting for another character’s turn. In other situations, it’s a poor choice, like when the extra damage is wasted because a more accurate attack would kill the enemy anyway. Chance determines the results of Deathblow, but the player has the choice of deciding if leaving the situation to chance is even necessary.

Examples of accepting/rejecting the dice:
  • Choosing to open or ignore a treasure chest that might be boobytrapped
  • Deciding between taking a clearly-lit path or a dark one
  • Fighting the urge to gamble all your money away on a chocobo race

Natural 20, dinosaur dies

Parallel Results

Finally, one of my favorite uses of randomness is parallel results. A dice roll that results in one of two or more different-but-equal outcomes has parallel results. An example would be opening a chest and receiving either a magical staff or a stout shield. Assuming both are roughly equal in potential usefulness, each result can lead the player to making different choices and adjusting their playstyle accordingly. The staff encourages them to make a more wizard-like character while the shield encourages them to play defensively, and this is where skill comes in. A situation like this asks the player to be versatile, asks them to master both the staff and the shield, and asks them to be ready to play any hand they’re dealt.

Examples of parallel results:
  • Receiving a random power-up or buff
  • Overworld battles with varying enemy arrangements
  • Weather that alters what creatures appear, what items can be found, etc. (ex. Animal Crossing)

These are just a few of the ways chance can be reconciled with skill and can, in fact, be used to create new kinds of skill-based gameplay. Don’t let anyone tell you the two are exclusive!

Excellent Uses of Randomness

If you’re going to include random elements in your game, you should know what they can accomplish and how best to incorporate them. The following games are some great uses of randomness and can be used as examples when looking for inspiration.

The Binding of Isaac

The Binding of Isaac is a remix on the original Legend of Zelda with psuedo-random elements at the very core of its gameplay. You travel through a basement dungeon, trying to get to the bottom. Playthroughs are short, difficult and often hopeless, but because the game is randomized and always changing, death is much less frustrating and replaying the game is exciting instead of boring. The game utilizes parallel results when presenting you with power-ups: every floor has (at least) two items that will increase your power, but which two you’re given are random, and it’s up to you to use them wisely to survive.

The Binding of Isaac also allows you to accept or reject the dice as you see fit. Although you have to rely on chance to progress, you get to choose the terms. You can decide whether you want to explore unmapped rooms in hope of more rewards, or if you want to take the safe path to the exit. If you find a pill, you can eat it for either a bonus or a punishment, but you also choose not to eat it at all. Your fate is at least partially in your hands. Taking too many risks may get you killed, but avoiding risks creates another risk - lack of power.

Finally, The Binding of Isaac also allows you to influence the dice. Certain items will increase your chance to find money, or hearts, or bombs, and that allows you to tilt the odds in your favor. All in all, this game is a great example of chance-done-well, and even if your game doesn’t use randomness to this extent, analyzing the relevant snippets of its design will do you well.

These chests probably contain spiders but you're going to open them anyway


Spelunky is to Mario what The Binding of Isaac is to Zelda. The game takes psuedo-random elements and jumbles them up into unique mazes every time you play, creating truly one-of-a-kind maps that don’t matter because you’re going to die in less than thirty seconds.

And that’s the beauty of it. In Spelunky, you die fast and get up faster. While each playthrough may end in failure, you learn a little bit each time: when to jump, when to attack, when to dodge. The elements are always the same - spiders that drop on you from above, arrow traps that sense movement, paranoid shopkeepers that protect valuable goods - but their arrangement is not. Each adventure is be unique, but you begin with the accumulated knowledge of all your past defeats.

While Spelunky incorporates a little bit of the previously mentioned techniques, where it really excels is maximizing replayability and suspense. You never know what’s going to be around the next corner, and you can only hope your experience will lead you to victory.

Being stuck between four traps at once is Spelunky 101


The puzzle game, Tetris is a great example of how randomness and skill can entwine to create a fun game that people play for hours (days, months, years). Weird shapes are constantly thrown your way and you have to be fast enough to deal with them, and some of them are much more useful than others.

Depending on what version of Tetris you’re playing, the pieces may be really random or only sort of random, but you can be certain that you will always be getting pieces you don’t want, and what separates the professionals from the amateurs is not how you handle the best blocks, but how you handle the worst.

Anyone can handle the Line, but few can manage the Z.


That wraps up my introduction to randomness in game design. If you’ve never messed with it before, I hope this helps you understand the basics, and if you’re experienced I hope it helps provoke some new ideas! Random elements can create wildly unique, ever-changing, ever-evolving games, and they are a fantastic tool in any designer’s toolbelt.


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Great read, slashphoenix, really interesting stuff.

I guess designing a game that’s full of randomness or non-randomness is all right as long as the game is fun and entertaining to play. I guess some players might have a problem with a game generally relying mostly on luck to progress through it (such as playing a board game / card games) but some people really don’t mind it so much; I guess it just depends on the audience.

I generally like playing a lot of luck based games; and I also don’t mind playing games that focus more on skill rather than luck. Either way, as long as it’s used properly then it shouldn’t be much of a problem.
Thanks :)

Yea, I think how much a game should rely on chance or skill has to be decided on a case-by-case basis, depending on what the designer is going for. Both kinds of play are fun for different reasons (and both are fun to design too)!
You the practice of self-promotion
Randomness does have a place in games that don't rely on pure skill. I do like your idea of rewarding players willing to take chances (or making them pay, greedy fools!).
Attributing BoI's RNG to dice is doing it some disservice I think! There's a lot that goes behind how the game deals with randomness that it is possible to manipulate once you have an idea of how it works.

Recommended watching if you really want a 'quick' glance at how it all pulls together. Timestamps where the juicy bits are in the video description along with a link to the pastebin with all of the findings of analyzing the code and understanding of how the game operates.
Randomness does have a place in games that don't rely on pure skill. I do like your idea of rewarding players willing to take chances (or making them pay, greedy fools!).

Exactly! As long as they know what they're getting into they have no one else to blame!

Attributing BoI's RNG to dice is doing it some disservice I think! There's a lot that goes behind how the game deals with randomness that it is possible to manipulate once you have an idea of how it works.

Oh yea, The Binding of Isaac is astoundingly complex in how it entwines luck and skill - a new player will bungle their way through a dungeon blind, but someone with experience can make accurate assumptions about what'll be next and twist it to their advantage. I remember you showing me the pastebin a while back and it still amazes me how many ways you can manipulate everything. BoI is one of the absolute best examples of skill-from-randomization I've ever seen.
"My father told me this would happen."
BoI is one of the absolute best examples of skill-from-randomization I've ever seen in a video game, bar none.


I have actually never played Bubble Bobble!
Awesome article, slashphoenix! I loved your explanations. I never knew that professional games can manipulate random elements to actually emphasize skill-based gameplay. I used to believe that random elements in games destroy skill, but this article proves me otherwise. Good read. :) One question though, what is your stance on purely skill-based games? A good example would be Tekken, Soul Calibur, and some PVP games like Bloodline Champions? Do you think they lack a sense of randomness in their gameplay and would benefit from more random elements, or do you think that games like that can function almost completely without any random elements needed at all?
Thanks! I'm glad you liked it!

There's definitely room for 100% non-random games and they can be a lot of fun. Most adventure games are totally non-random (Zelda, Mario) and they're still a blast to play and learn the first time. Another example is chess! It's one of the most popular and well-known games in existence, and there's zero randomness to the play; the game setup is always the same and both players are always on equal ground, yet chess strategies have evolved for centuries.

I think the biggest difference between random and non-random games is not necessarily how much skill they require, but what kind of skill they require. Non-random games like Chess, Mario or Tekken tend to favor those who have been playing the longest and have memorized the most tactics. Games with random elements like Tetris favor quick thinking and versatility - you're given a problem and you have to wing it, often with less-than-optimal tools. In a way, random elements add a sense of crisis management to games.

As a sidenote, since Tekken and chess both are usually played against other people, they have a sense of randomness to them as well, because other people aren't immediately predictable. Part of the fun of a long-time 2-player grudge match is learning how your opponent plays and reacting accordingly... so there's never a perfect choice, because your strategy is always evolving.
You've made some interesting points. Players do, in a sense, seem somewhat random and unpredictable. Random games tend to add in an unpredictable type of atmosphere that makes players adapt to different situations that can benefit them in the long run. Less random games tend to focus mainly on experience and learning.

A really successful game that I consider having almost no random elements at all is Cooking Mama for Nintendo. All their levels and their recipes and completely laid out in the table for you. It seems pretty difficult and daunting the first time you try it, but over time, you can learn how to time your stylus swipes to get the higher score. The reason for their success, I believe is the familiarity of the theme, the uniqueness of their gameplay, and their user-friendly interface for kids.

There are many other good examples of games that use random elements and non-random elements, but overall, I agree with what you're saying. It doesn't matter how random or non-random a game is, what matters is how those elements were put together to create an enjoyable atmosphere for the player.
A minor point to bring up about "RNG or not" is that a sufficiently complex, but non-random system can be as unpredictable as RNG to a new player, while still being a manipulable system that's rewarding to learn. An example would be a golf game - you can get better at predicting how wind, terrain, and spin/power levels will affect the ball's trajectory, but most people will never learn to fully predict it.
(PS: 100% accurate Sniper CR + Deathblow is hilarious)
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