# Worlds of Design: Always Tell Me the Odds

If GMs (and game designers, and gamers) understand “the odds” they will be able to make better choices and understand why some things happen in their games - and some don’t.

Picture courtesy of Pixabay.
Never tell me the odds!
--Han Solo (Star Wars)​

Most people don't understand odds and randomness in the most simple dimensions, especially when you're talking about dynamic odds.
--Keith S. Whyte. Executive Director. National Council on Problem Gambling​

We often hear about “the percentages” and “the odds” in sports. For example, the odds for the home team winning (regular season: NBA 59.9%, NFL 57.1, NHL 55.1, MLB 54.0, MLS soccer (where there are draws) home win ratio of 49.4 percent over a 15 year period, compared to just 26.5 percent away wins). Though game design does not require higher math, game designers need to know simple arithmetic and probability. There are some odds we can talk about in RPGs, as well, and about how people react to those odds.

The notion that it can be a "fair fight" in an RPG? 50/50? Nope.

How much is a fight biased toward the adventurers? Let’s consider the NCAA Basketball tournament. Let’s say that a team is so good, it can win 90% of its games against the better teams, the ones in the tournament. This is unlikely: how many teams have a season record as good as 27-3 (90%) though they’re playing weak as well as strong teams? When you’re playing the stronger teams, 90% is quite unlikely. But let’s use that anyway.

So what are the chances of winning the tournament (six games in a row) even with that 90% (beyond-likelihood) capability?

 90%​ win 1 in a row​ 81.00%​ win 2 in a row​ 72.90%​ win 3 in a row​ 65.61%​ win 4 in a row​ 59.05%​ win 5 in a row​ 53.14%​ win 6 in a row​

Even that most unlikely team that can win 90% of games against tournament-quality opposition, only has a 53.14% chance of winning the tournament. Even a team with a 99% win likelihood wins the six-game tournament only 94.15% of the time (“fail on a roll of 1 on d20").

(How is this calculated? You multiply, you don't add. So to win three games in a row, it’s 90% times 90% times 90%.)

This is why the “best team” often fails to win the tournament. This is why some pro sports play seven-game playoff series, in the hope that luck “evens out” and the better team will win.

Translate This into RPGs

Extrapolate that into RPG sessions with perhaps one big battle per session, or maybe more! Practically speaking, either you need really astute players willing to run away from almost any encounter, in order to avoid taking chances, or you need to arrange a huge bias in favor of the players in a typical encounter. Or they're going to lose and possibly die pretty soon.

Go back to the tournament example. If the players are 90% likely to win, after six encounters there will be around a 47% chance that they will have lost one of those encounters.

The whole notion of RPG combat as "sport", as something that's "fair", is nonsense in light of these calculations.

Playing Styles

Some play for "the rush", for glory, and like Han Solo don't want to know the odds before they do something. If you accommodate them, then the bias in favor of the players must be even greater, or you'll have dead characters in no time. (This brings up the question of "fudging" dice rolls in favor of characters, which I may address another time. Some GMs do it routinely, others never.)

Is it fun to play to survive, to “win”, instead of for glory? Depends on the person. It is for me, when I expand it to include survival for the entire group, not just my character(s). In contrast, in the late 70s I played in a game that was supposed to act as the stimulus for someone to write a story. I tried to do something "heroic". My character got dead.

Many gamers don't understand probability, and so over- (or under-) estimate their chances of success. Some don't understand the scope of the chances. 1 in a thousand vs 1 in a million is a massive difference, but people often don't see it that way. It's yet another case of perception not matching reality.

That's where we get those who don't understand odds, who think that anything (no matter how outlandish) ought to be possible once in 20 (a 20 on a d20) or at worst once in a hundred (100 on percentage dice). No, the chance of most anything happening in a given situation are astronomically against. (And "astronomically" is practically the same as "impossible".)

Recently I talked with a gamer who is very skeptical of probabilities, but doesn't understand them. He thought it was terribly unlikely that a player could roll five dice in a row and get at least a 4 on every roll. The chances, 50% to the fifth power, amount to better than 3%. For some reason he thought that rolling the dice successively rather than altogether made a difference - nope, what's come before has no bearing on what comes after, in odds. And what about five 1's in a row? That's 16.66% (a 1 on a d6) to the fifth, .000129 or .0129%. One tenth of one percent (one chance in a thousand) is .01%. So slightly better than one chance in a thousand. Rolling seven 1's in a row is about 3.5 chances in a million. Or perhaps more easily, rolling a 1 on every one of six 10-sided dice is a one-in-a-million chance.

To summarize: For designers, fudging the dice (or the quality of the opposition) is inevitable. For players, it helps to understand probabilities in games

Reference: James Ernest (Cheapass Games) - Probability for Game Designers | League of Gamemakers

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### Lewis Pulsipher

Dragon, White Dwarf, Fiend Folio

#### Fenris-77

##### Small God of the Dozens
Supporter
Nice article. I feel the lack of a bell curve diagram though. People always feel like they know where they're at with a bell curve. It gives them a comfortable sense that the abyss of probability isn't actually staring back at them, but just inviting them over for a nice cup of tea.

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#### Jacob Lewis

##### Ye Olde GM

Yeah, that's my actual ship... er, vehicle!

#### Ovinomancer

##### No flips for you!
This is why the “best team” often fails to win the tournament. This is why some pro sports play seven-game playoff series, in the hope that luck “evens out” and the better team will win.
Pedantic quibble and pet peeve: No, probabilitues are NEVER a cause of anything. Teams lose for specific reasons, not probabilities. Probabilities are models that can help inform decision making, but do not have any impact on actual outcomes. Else the sports betting industry would be terribly boring.

#### GMMichael

##### Guide of Modos
The whole notion of RPG combat as "sport", as something that's "fair", is nonsense in light of these calculations.

To summarize: For designers, fudging the dice (or the quality of the opposition) is inevitable. For players, it helps to understand probabilities in games
Is there an implied "traditional" in "RPG combat?" Because some games don't treat combat as sport. Some games treat it as a gruesome, dirty, blood-spitting disaster (thanks, Zweihaender).

I wouldn't say that dice-(or opposition)-fudging is inevitable. I'm reading that to mean that fudging must sometimes be used to allow PCs to win a contest, so designers shouldn't stress too much about making a fair game. But what if the game's rules aren't set up to determine win/loss? Or what if the game allows the GM to write "victory" into the script, before the battle has begun? Or it has a built-in fudging rule (like fate points) that players can use? It's not "fudging" if there's a rule for it!

A follow-up article to this might discuss why GMs should understand the odds behind specific dice-contests, because as the ones with the primary responsibility for modifying those contests, GMs should know the fire with which they're playing. E.g. how adding a bonus point to a roll in D&D is not equal to adding a bonus point to a Fantasy AGE roll.

PS - Totally agree that PCs shouldn't expect to win every battle. My characters usually like to avoid combat, in a sort of "I like living" way!

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#### clearstream

##### (He, Him)
Pedantic quibble and pet peeve: No, probabilitues are NEVER a cause of anything. Teams lose for specific reasons, not probabilities. Probabilities are models that can help inform decision making, but do not have any impact on actual outcomes. Else the sports betting industry would be terribly boring.
I had a similar quibble, but decided the analogy was only there to illustrate compound odds, not to claim any virtue as a simulation.

Still, I think explaining compound odds isn't all that helpful. Most people struggle to really grasp what it means to have a non-zero chance of failure in the first place. I find myself asking people to first of all think about a coin-flip. And then think about what kinds of things they might be willing to risk on a coin-flip. And then think about possible worlds, where each side of the coin lands us in a different possible world. I then talk about near-certainties, and what that might feel like. And so on.

Working along those lines seems to gradually bring people to understand odds.

#### clearstream

##### (He, Him)
Is there an implied "traditional" in "RPG combat?" Because some games don't treat combat as sport. Some games treat it as a gruesome, dirty, blood-spitting disaster (thanks, Zweihaender).

I wouldn't say that dice-(or opposition)-fudging is inevitable. I'm reading that to mean that fudging must sometimes be used to allow PCs to win a contest, so designers shouldn't stress too much about making a fair game. But what if the game's rules aren't set up to determine win/loss? Or what if the game allows the GM to write "victory" into the script, before the battle has begun? Or it has a built-in fudging rule (like fate points) that players can use? It's not "fudging" if there's a rule for it!

A follow-up article to this might discuss why GMs should understand the odds behind specific dice-contests, because as the ones with the primary responsibility for modifying those contests, GMs should know the fire with which they're playing. E.g. how adding a bonus point to a roll in D&D is not equal to adding a bonus point to a Fantasy AGE roll.
I wrote an article a while back about stakes. Odds mean nothing without stakes. And the stakes in games follow some interesting patterns.

#### atanakar

##### Hero
I never fudge the dice as a DM. Everything is rolled in front of the screen. What's the point of having odds if the GM cheats all the time. PCs should fail or succeed on their own merit or lack of.

As a DM I feel it's my responsabilty to explain odds of success before a Player wants his PC to do something very unlikely to succeed. I call that a Wisdom 3 moment. If the player persists he will live with the consequences.

#### univoxs

##### That's my dog, Walter
Supporter
A trope I dislike in traditional TTRPG is that every foe is meant to be fought or else would not be placed before the characters. It is gerenally assumed by the party that every monster is somewhat appropriate for their current power level. Anyone with experiance in WoD games sees this a different way. I encourage my players to talk first stab later, no matter the game.

#### Blue

##### Ravenous Bugblatter Beast of Traal
I'm with Lewis on this one - most people don't actually understand probabilities. I will often see common sense - and completely wrong - assumptions. From easy things like not understanding bell curves and thinking d20 is just like 3d6 but with two extra/less at either end, to not understanding the actual odds of X successes before Y failures, to more in depth ones.

A challenge of design is to make these clear. "d20+modifiers >= DC" isn't hard to grasp, fudges 4dF even easier while a die pool with a required number of successes may mask probability from the casual gamer. Especially when balancing tryign to let the dice tell the whole story - an 11 doesn't say if you succeed or fail, but some systems have penalty dice built in so that the roll determines success or failure without any additional manipulation - but often at the price of obfuscating the odds even more.

#### Jaeger

##### That someone better
Is there an implied "traditional" in "RPG combat?"

Yes, whatever the current edition of D&D is. Market leader sets the tone.

I wouldn't say that dice-(or opposition)-fudging is inevitable. I'm reading that to mean that fudging must sometimes be used to allow PCs to win a contest,...

Which is utterly abhorrent of course.

Fudging rolls defeats the whole purpose of the "GAME" in RPG.

A follow-up article to this might discuss why GMs should understand the odds behind specific dice-contests, because as the ones with the primary responsibility for modifying those contests, GMs should know the fire with which they're playing. ...

This!

I never fudge the dice as a DM. Everything is rolled in front of the screen. What's the point of having odds if the GM cheats all the time. PCs should fail or succeed on their own merit or lack of.

As a DM I feel it's my responsabilty to explain odds of success before a Player wants his PC to do something very unlikely to succeed. I call that a Wisdom 3 moment. If the player persists he will live with the consequences.

And this!

#### Mortus

##### Explorer
I ask my players during session 0 if they want my rolls private or public. The only exception is if the character would be unaware of their success like looking for traps. If private, I fudge rolls to better guide the story or to counteract really bad luck.

In my experience all public rolls usually have exceptions too. In a 1E Pathfinder Society game a few GenCons ago, I was playing a 5th level half orc barbarian that I had just leveled up. I was so impressed with his high hit points. In the opening scene we were surprised attacked by mounted warriors with lances. My PC was hit with a crit and killed outright. The GM was kind enough to just leave my PC bleeding and I was bandaged up in the same round.

I think both styles have there pros and cons and I enjoy both.

In the weekly 5E Decent into Avernus I’m playing currently the DM Is rolling publicly. Last game we had a long combat and his luck with the dice was astoundingly bad and we were on fire. At one point in the battle, my buddy took a poorly crafted Molotov cocktail from me to throw on some barricades we had made and I asked the DM for a 1% chance that it would ignite. Of course, my buddy rolled a 1 on 1d100.

#### R_J_K75

##### Hero
I never fudge the dice as a DM. Everything is rolled in front of the screen.

I fudge rolls very little, though I do occasionally, but I generally don't use a screen either, but with playing online now everyones on the honor system. I started gaming with my neighbor a few years back and he made a comment that it seemed to him that they were winning fights that they probably shouldn't have and things were kind of easy. Even then I wasn't fudging dice much, and I've since stopped. I've purposely started making combat encounters pretty difficult ever since. Last combat I ran lasted 2 sessions and went 11 rounds with the players leveling up between sessions. If they hadn't I think they probably would have died, I think I rolled about 5-6 natural 20s last game, 2 of which were in a row and brought the barbarian down to 2 hp.

A trope I dislike in traditional TTRPG is that every foe is meant to be fought or else would not be placed before the characters. It is gerenally assumed by the party that every monster is somewhat appropriate for their current power level.

I don't use random encounters much anymore nor do I map out areas proactively in my games either. When I did though I definitely would place creatures well outside of the characters ability to defeat here and there. I would encourage the players to research with the locals before adventuring to get an idea of whats in the area. Even then I would mix in both truth and fiction so that no matter how prepared they were there were still some surprises. I also treated most creatures as unique and gave them tweaks here and there to make player victory not as assured, but more rewarding when they did win. Van Richtens Guides to Monster Hunting was really good for ideas on how to do this.

#### hawkeyefan

##### Legend
I had the odds at 10 to 1 this thread would be about rolling dice. I was way off.

#### Nagol

##### Unimportant
If GMs (and game designers, and gamers) understand “the odds” they will be able to make better choices and understand why some things happen in their games - and some don’t.

Never tell me the odds!
--Han Solo (Star Wars)​

Most people don't understand odds and randomness in the most simple dimensions, especially when you're talking about dynamic odds.
--Keith S. Whyte. Executive Director. National Council on Problem Gambling​

We often hear about “the percentages” and “the odds” in sports. For example, the odds for the home team winning (regular season: NBA 59.9%, NFL 57.1, NHL 55.1, MLB 54.0, MLS soccer (where there are draws) home win ratio of 49.4 percent over a 15 year period, compared to just 26.5 percent away wins). Though game design does not require higher math, game designers need to know simple arithmetic and probability. There are some odds we can talk about in RPGs, as well, and about how people react to those odds.

The notion that it can be a "fair fight" in an RPG? 50/50? Nope.

How much is a fight biased toward the adventurers? Let’s consider the NCAA Basketball tournament. Let’s say that a team is so good, it can win 90% of its games against the better teams, the ones in the tournament. This is unlikely: how many teams have a season record as good as 27-3 (90%) though they’re playing weak as well as strong teams? When you’re playing the stronger teams, 90% is quite unlikely. But let’s use that anyway.

So what are the chances of winning the tournament (six games in a row) even with that 90% (beyond-likelihood) capability?

 90%​ win 1 in a row​ 81.00%​ win 2 in a row​ 72.90%​ win 3 in a row​ 65.61%​ win 4 in a row​ 59.05%​ win 5 in a row​ 53.14%​ win 6 in a row​

Even that most unlikely team that can win 90% of games against tournament-quality opposition, only has a 53.14% chance of winning the tournament. Even a team with a 99% win likelihood wins the six-game tournament only 94.15% of the time (“fail on a roll of 1 on d20").

(How is this calculated? You multiply, you don't add. So to win three games in a row, it’s 90% times 90% times 90%.)

This is why the “best team” often fails to win the tournament. This is why some pro sports play seven-game playoff series, in the hope that luck “evens out” and the better team will win.

Translate This into RPGs

Extrapolate that into RPG sessions with perhaps one big battle per session, or maybe more! Practically speaking, either you need really astute players willing to run away from almost any encounter, in order to avoid taking chances, or you need to arrange a huge bias in favor of the players in a typical encounter. Or they're going to lose and possibly die pretty soon.

Go back to the tournament example. If the players are 90% likely to win, after six encounters there will be around a 47% chance that they will have lost one of those encounters.

The whole notion of RPG combat as "sport", as something that's "fair", is nonsense in light of these calculations.

Playing Styles

Some play for "the rush", for glory, and like Han Solo don't want to know the odds before they do something. If you accommodate them, then the bias in favor of the players must be even greater, or you'll have dead characters in no time. (This brings up the question of "fudging" dice rolls in favor of characters, which I may address another time. Some GMs do it routinely, others never.)

Is it fun to play to survive, to “win”, instead of for glory? Depends on the person. It is for me, when I expand it to include survival for the entire group, not just my character(s). In contrast, in the late 70s I played in a game that was supposed to act as the stimulus for someone to write a story. I tried to do something "heroic". My character got dead.

Many gamers don't understand probability, and so over- (or under-) estimate their chances of success. Some don't understand the scope of the chances. 1 in a thousand vs 1 in a million is a massive difference, but people often don't see it that way. It's yet another case of perception not matching reality.

That's where we get those who don't understand odds, who think that anything (no matter how outlandish) ought to be possible once in 20 (a 20 on a d20) or at worst once in a hundred (100 on percentage dice). No, the chance of most anything happening in a given situation are astronomically against. (And "astronomically" is practically the same as "impossible".)

Recently I talked with a gamer who is very skeptical of probabilities, but doesn't understand them. He thought it was terribly unlikely that a player could roll five dice in a row and get at least a 4 on every roll. The chances, 50% to the fifth power, amount to better than 3%. For some reason he thought that rolling the dice successively rather than altogether made a difference - nope, what's come before has no bearing on what comes after, in odds. And what about five 1's in a row? That's 16.66% (a 1 on a d6) to the fifth, .000129 or .0129%. One tenth of one percent (one chance in a thousand) is .01%. So slightly better than one chance in a thousand. Rolling seven 1's in a row is about 3.5 chances in a million. Or perhaps more easily, rolling a 1 on every one of six 10-sided dice is a one-in-a-million chance.

To summarize: For designers, fudging the dice (or the quality of the opposition) is inevitable. For players, it helps to understand probabilities in games

Reference: James Ernest (Cheapass Games) - Probability for Game Designers | League of Gamemakers

It gets really bad when mechanics become opaque and improperly explained. The original skill challenges in 4e were like this. Markov chain probabilities are not obvious to most folk. It was obvious when one did the math that the original designers hadn't. And that's before the inclusion of auto-fail options in the scenario like the example SC had. The odds of success without the DM doing something 'off-book' were terrible.

#### Hussar

##### Legend
I wrote an article a while back about stakes. Odds mean nothing without stakes. And the stakes in games follow some interesting patterns.

Personally, I think this, right here, is probably one of the biggest table issues that I see. DM's that don't really understand how odds work that then try to cobble together in game elements where the reward (the stakes) is so much smaller than they should be. Or, worse yet, cobble together a situation where they don't seem to understand that multiple checks result in much greater chances of failure, particularly if each failed check results in catastrophic failure.

Take the old saw about sneaking around and scouting. I've seen DM's (and, honestly, BEEN the DM unfortunately) just keep repeatedly calling for checks until the character/group fails and then sit back and say, "Well, you had the chance to sneak past, you just failed". Neverminding that if you force, say 5 checks and the party has a 25% chance of failure, then failure is pretty much guaranteed.

The reward in a game MUST reflect the risk. And this is something that you see over and over again that DM's just don't grasp.

#### prosfilaes

A trope I dislike in traditional TTRPG is that every foe is meant to be fought or else would not be placed before the characters. It is gerenally assumed by the party that every monster is somewhat appropriate for their current power level. Anyone with experiance in WoD games sees this a different way. I encourage my players to talk first stab later, no matter the game.

I feel the first part of the article was quite relevant here. If there's a 10% chance of the party dying in any particular battle, they won't last long. In WoD, there's setting features to discourage the bad guys from just murdering the PCs. In D&D, there's creatures that have no reason not to murder any PCs that get near them. Demon lords just aren't going to let a good party get away, and wraiths aren't going to let a living party get away. There's ways to do that without just wiping out the party (and I suspect 5E, which I have limited experience with, might be more merciful than editions where parties raise in power more dramatically), but it's easy to have the party open a door and be dead to an opponent they had no chance of defeating or running from.

#### Shiroiken

##### Hero
A trope I dislike in traditional TTRPG is that every foe is meant to be fought or else would not be placed before the characters. It is gerenally assumed by the party that every monster is somewhat appropriate for their current power level. Anyone with experiance in WoD games sees this a different way. I encourage my players to talk first stab later, no matter the game.
In early editions of D&D, there was a chart that would determine the mood of the opposition, allowing for potential negotiations or even just passing each other by. Sometimes it was unrealistic, but that's what the DM's for, to know when to use the chart or ignore it. Of course, in those same editions, killing stuff was a lot less of your xp, so it just wasn't worth it unless necessary.

As for everything should be fought... that's a mentality I've tried to break from a lot of 3E and 4E players. I ran LMoP to teach 5E to an experienced group of 3E players, and they decided to fight the dragon at level 3, where they were immediately killed. One of them asked how you were supposed to kill the dragon, so I asked "what indicated you should even seek out the dragon?" They just heard about the dragon and assumed they were supposed to kill it.

There's ways to do that without just wiping out the party (and I suspect 5E, which I have limited experience with, might be more merciful than editions where parties raise in power more dramatically), but it's easy to have the party open a door and be dead to an opponent they had no chance of defeating or running from.
This is a failure of adventure design. If you're going to have a party run across something that's way beyond their power level, there should either be hints beforehand (warning them to plan appropriately to avoid or flee) or it should be set up in such a way as to allow the PCs to see the enemy before they can be seen. No one wants a TPK just because of a bad roll on a random encounter table or because they had no idea the cult had managed to summon a demon.

We just ran a Midguard one shot, and most of the adventure was pretty good, but there were absolutely NO clues about what was going on. We reacted to events, trying to find out what might be going on, but instead we had to face a CR 5 as four level 1 PCs. We only pulled it off because we figured out its immediate goal and its weakness (fire), and then were able to keep it from pulling off its goal while in a burning building (we were in it too). My character died heroically making sure the creature didn't escape, which was fine for a one shot, but would have sucked for the start of a campaign. Since we figured we defeated the enemy, we followed its tracks back to its lair in hopes of finding out what was going on. Instead we got hit with a CR 6 creature that had a summoning power, with no warning or chance to escape... TPK. We talked with the DM about it later, and it turns out the only way to figure out what's going on is to kill the CR 5, then reduce the CR6 to 1/4 HP, where it will surrender and explain. Most of the this one shot was really good, as we just assumed we missed something as players, but no it was a deathtrap adventure that really gave the players no chance of success.

#### CapnZapp

##### Legend
As far as I understand it, @lewpuls ,the opaque odds of dice pool games (e.g. Vampire) was intended to be a feature not a bug. That is, people play that game because they don't want to know the (exact) odds.

As someone with a fair grasp of maths and statistics, I have never understood that sentiment. I have always preferred the percentile die as the best resolution system (e.g. Basic Role-Playing) precisely because it makes it very easy to see the odds. (Assuming you understand percentages, of course)

But I guess (and this is not intended to be a personal insult to anyone) if you don't do math, playing a game with opaque probabilities (figuring out the odds in dice pool games is fiendishly difficult) evens out the odds compared to a friend that do math...

#### CapnZapp

##### Legend
most people don't actually understand probabilities.
I would take a more modest position:

most people don't actually understand how hard people find probabilities.

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