Advantage & Disadvantage Probability

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KrakaJak

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This article does a pretty good breakdown of Advantage and Disadvantage from a probability standpoint:

Advantage in D&D 5

I am not a fan of Advantage/Disadvantage in play. I thought it was a cool idea on paper, but it always felt really swingy and unreliable as either a bonus, or a penalty. The math here shows that it IS really swingy, where the probability boost is equivalent to +/-1 to +/- 5 depending on where the dice need to land and pushes the results towards the median rather than to the edges. For me, this confirms my own bias against the mechanic, but I think the math was interesting and worth discussion.
 
Worth noting that Advantage almost doubles your chances of a crit, which is a strong bonus in 5e.

My favourite part of the mechanic is it's feel, though; it feels good to roll two dice and throw away the worst. It feels bad to roll two dice and throw away the best. And because they're rolled at the same time, it's more immediate and obvious than something like re-roll effects.
 
I love the system. It takes all the fiddly bits and puts them into the player's hands.
It makes a tossup a sucker bet for or against, and the swingyness has an appeal to me.

Player complains that they should have a better shot at sonething. Sure... solid case, take advantage. No arguing over how fair the modifiers are. Just roll and its all on you.
 
My favourite part of the mechanic is it's feel, though; it feels good to roll two dice and throw away the worst. It feels bad to roll two dice and throw away the best. And because they're rolled at the same time, it's more immediate and obvious than something like re-roll effects.
Agreed. I know that the maths are sometimes a little wonky, but the notion of rolling two dice and taking the best (or worst) is a nice one. The rule feels good and is simple to use.
 
The math here shows that it IS really swingy, where the probability boost is equivalent to +/-1 to +/- 5 depending on where the dice need to land and pushes the results towards the median rather than to the edges.
I don't understand the complaint because the d20 has a swingy flat curve in the first place. Low level combat can be crazy swingy. If you want bell curve probability you swap the d20 for 3d6 and handle advantage/disadvantage through the ol' roll 4d6 and drop lowest/highest.
 
For anybody wondering how the chances work out in Mongoose Traveller 2E.
Normal is 2d6
Advantage is keep best two from 3d6
Disadvantage is keep worst two from 3d6

TargetAdvantageNormalDisadvantageAdvantage GainDisadvantage Loss
2​
100.00​
100.00​
100.00​
0.00​
0.00​
3​
99.54​
97.22​
92.59​
2.31​
-4.63​
4​
98.15​
91.67​
80.09​
6.48​
-11.57​
5​
94.91​
83.33​
64.35​
11.57​
-18.98​
6​
89.35​
72.22​
47.69​
17.13​
-24.54​
7​
80.56​
58.33​
31.94​
22.22​
-26.39​
8
68.06
41.67
19.44
26.39
-22.22
9​
52.31​
27.78​
10.65​
24.54​
-17.13​
10​
35.65​
16.67​
5.09​
18.98​
-11.57​
11​
19.91​
8.33​
1.85​
11.57​
-6.48​
12​
7.41​
2.78​
0.46​
4.63​
-2.31​
 
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Almost every dice mechanic has some kind of problem if there's any modifiers that make it so everyone doesn't have the same chance of success on a single die roll. Anything that brings in multiple rolls (whether it's some kind of attrition system or some kind of re-roll system) will introduce oddities. Modifiers that mean not everyone has the same target number have oddities which vary depending on whether the dice are a single flat probability (1d6, 1d20, 1d100, etc) or some kind of curve (2d6, 4dF, Normal Distribution*, etc.), how much variation the modifiers are, and how far the probability is from a middle of the road 50-50.

* I claim using the actual normal distribution for open ended bell curve is superior. That's really only from some kind of pure look. The truth is the Cold Iron system that I learned how to use the normal distribution with is either quite swingy or gives a much bigger advantage to having +1 over your opponent than you would expect.
 
I don't understand the complaint because the d20 has a swingy flat curve in the first place. Low level combat can be crazy swingy. If you want bell curve probability you swap the d20 for 3d6 and handle advantage/disadvantage through the ol' roll 4d6 and drop lowest/highest.
Because the equivalent bonus is on a bell curve, when the roll probability of regular play is flat. If I give a +2 bonus, that is a flat 10% bonus to all results. A player is guaranteed to roll between a 3 and a 22 (a 22 cannot be rolled with advantage, so characters cannot exceed previous limitations with advantage/disadvantage either).

Combining the results of dice (like 3d6) gives a bell curve toward the mean. Bonuses and penalties in GURPS are swingy as fuck because of that. A +1 bonus
to someone with a skill of 9 means a whole lot more to than it does to someone with a skill rating of 3, or16 in Gurps. Introducing that swinginess to what you call an "already swingy" d20 roll compounds the issue.

How this works out in play:
A character with a decent skill bonus in a check is barely benefitted at all by advantage...and disadvantage is actually a huge drawback.
The reverse, a character with a poor skill bonus in a check is greatly benefitted by advantage and disadvantage is barely a drawback.

A +2 bonus hinders and hampers both characters equally.
 
Because the equivalent bonus is on a bell curve, when the roll probability of regular play is flat. If I give a +2 bonus, that is a flat 10% bonus to all results. A player is guaranteed to roll between a 3 and a 22 (a 22 cannot be rolled with advantage, so characters cannot exceed previous limitations with advantage/disadvantage either).

Combining the results of dice (like 3d6) gives a bell curve toward the mean. Bonuses and penalties in GURPS are swingy as fuck because of that. A +1 bonus
to someone with a skill of 9 means a whole lot more to than it does to someone with a skill rating of 3, or16 in Gurps. Introducing that swinginess to what you call an "already swingy" d20 roll compounds the issue.

How this works out in play:
A character with a decent skill bonus in a check is barely benefitted at all by advantage...and disadvantage is actually a huge drawback.
The reverse, a character with a poor skill bonus in a check is greatly benefitted by advantage and disadvantage is barely a drawback.

A +2 bonus hinders and hampers both characters equally.
That seems closer to reality to me actually.
The more competent you becomd the more set you are. Adversity hinders your routine more.
The greener you are the less competsnt you are but the more you rely on luck and wits because everything is adversity.
In a game with such a huge power curve this works well to equalize things.
 
see, I'm a fan of advantage/disadvantage. I think it does well, in this context. What's more is that they don't stack - 1 advantage takes out any number of disadvantages and vice versa. It also disincentivizes stacking of bonuses, as you lose benefit from advantage as you go up in skill. it's a way of signaling "go look somewhere else for an advantage". You view this negatively, but I say it's a positive. it also encourages fighting more highly skilled opponents by imposing disadvantage on them as a way to negate their skill.

Now, I will agree it should not be mixed with static bonuses ala Bless. This does skew things pretty weirdly. Bless always struck me as out of place in 5e for this reason.

In mythras, I will occasionally hand out advantage/disadvantage instead of an Easy bonus or a Formidable penalty. The chances are very close, and the mechanic prevents the need for any mental calculations. It also maps to a luck point based reroll, so I can describe those as Inspirations if I want to.
 
That seems closer to reality to me actually.
The more competent you becomd the more set you are. Adversity hinders your routine more.
The greener you are the less competsnt you are but the more you rely on luck and wits because everything is adversity.
In a game with such a huge power curve this works well to equalize things.

That's not my experience at all.

Let's look at +5 athletics, vs +0.



You are a faster runner, and you have a 10 mph wind is at your back during a time trial, I should have no chance to beat you barring some supremely bad luck on your part (a misstep twisting your ankle etc.). You are more competent, and you have the advantage...you should destroy me here. You having advantage, I still have a ~15% chance to beat (or tie) you, down from 25% all things being even.

Now I have a 10 mph tailwind....same advantage. My chance of beating you is now ~50%. The same advantage that gave you a 10% more likelihood of beating me, has doubled my chances to beat you and completely evened the odds.

Now +5 vs +5 athletics. Equally competent.

We both have a 50/50 chance of winning. You get a 10 mph tailwind, advantage. You have a 75% chance of beating me. Advantage has made you more likely to beat a trained runner, than it did an untrained runner. Advantage and Disadvantage are dumb.
 
That's not my experience at all.

Let's look at +5 athletics, vs +0.



You are a faster runner, and you have a 10 mph wind is at your back during a time trial, I should have no chance to beat you barring some supremely bad luck on your part (a misstep twisting your ankle etc.). You are more competent, and you have the advantage...you should destroy me here. You having advantage, I still have a ~15% chance to beat (or tie) you, down from 25% all things being even.

Now I have a 10 mph tailwind....same advantage. My chance of beating you is now ~50%. The same advantage that gave you a 10% more likelihood of beating me, has doubled my chances to beat you and completely evened the odds.

Now +5 vs +5 athletics. Equally competent.

We both have a 50/50 chance of winning. You get a 10 mph tailwind, advantage. You have a 75% chance of beating me. Advantage has made you more likely to beat a trained runner, than it did an untrained runner. Advantage and Disadvantage are dumb.
Hey KrakaJak KrakaJak I know where you are coming from. Part of that is baked into the system with to the flat probability of the d20 so you get stupid swingy results especially in contested skill checks. That's one good reason of many to minimize skill checks.

Furthermore I wouldn't grant advantage on Athletics for a tailwind. That is maybe worth a +1 and that's a maybe. It would have to be something big for me to grant Advantage, like the overdrive function on a cybernetic heart or a Potion of Pussiance.
 
I've never understood the obsession some people have with bell curves for resolution rolls. I've seen it recommended changing the attack roll in D&D to a 3d6 instead of a d20. Bizarre.
Anyway, not a fan of advantage/disadvantage myself. Mostly what I want from rules is sensible and easily understood calculations of probabilities. The more obfuscation there is, the harder it is to make appropriate decisions.
 
I've never understood the obsession some people have with bell curves for resolution rolls
The usual argument is about results clustering around the average rather than "terrible", "average" and "amazing" all being equally likely.
 
That's not my experience at all.

Let's look at +5 athletics, vs +0.



You are a faster runner, and you have a 10 mph wind is at your back during a time trial, I should have no chance to beat you barring some supremely bad luck on your part (a misstep twisting your ankle etc.). You are more competent, and you have the advantage...you should destroy me here. You having advantage, I still have a ~15% chance to beat (or tie) you, down from 25% all things being even.

Now I have a 10 mph tailwind....same advantage. My chance of beating you is now ~50%. The same advantage that gave you a 10% more likelihood of beating me, has doubled my chances to beat you and completely evened the odds.

Now +5 vs +5 athletics. Equally competent.

We both have a 50/50 chance of winning. You get a 10 mph tailwind, advantage. You have a 75% chance of beating me. Advantage has made you more likely to beat a trained runner, than it did an untrained runner. Advantage and Disadvantage are dumb.
I get its your opinion... but its far from dumb.

My experience time...
Fighting in the SCA.
All of the long time fighters have trained their muscle memory and react a certain way. The daze of combat is gone and they can clearly see the beginning of a swing before it happens. They clearly know their stuff and are equal to their peers. In a one on one fight its 50/50. They should clearly dominate beginner fighters and they usually do.

New fighters still get lucky shots... how come?

Well it comes down to chaos. A new fighter has no trained memory or precise movement. Their shots are wild and flailing around so reading their movements is harder. That wrap shot they start is... oh shit! Actually a J shot! How did he do that?!?
The vet fighter is locked in a set of assumptions from experience and thus defensless.

Edit: this is to illustrate tge normal curve with a base 5% chance of hit regsrdless of skill vs armor (roll a 20).

Now, the advantage. They are both fighting but behind the new fighter someone flashes a bright light at the pair. The veteran fighter gets disadvantage or the newbie gets advantage, and coupled with an already unpredictable opponent, he crashes and burns.
Reverse it and the vet gets advantage. The skill difference is already there so bring more able to land the shots is less of an advantage because they have already overcome the chaos of the fight anyway.

In fact, the advantage disadvantage play is normally played out via headgames. If i get in my opponents head I have a significantly higher chance to succeed over them at a similar skill level.

To me, this mechanic perfectly abstracts my life experience.

I I maneuver my back to the sun so it is in my opponents eyes. I now have a significantly better chance of winning.

I have a superior reach and force my opponent to close in more than me.

My armour is lighter, less cumbersoms and pinches less.

It goes on but sll of these are factors.

So your runners... both are equal, but one woke up with a cramp, or the runs. Maybe one got a dear john letter? You need a condition for the advantage it doesnt just happen.
 
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The usual argument is about results clustering around the average rather than "terrible", "average" and "amazing" all being equally likely.
There's probably some philosophical debate about negating the possibilities of amazings and terribles in service of averages, but I'd probably get too bored to pursue it for more than a lark.
 
There's probably some philosophical debate about negating the possibilities of amazings and terribles in service of averages, but I'd probably get too bored to pursue it for more than a lark.
Honestly its all arguing who has the better math to fit their vision on how an abstract version of luck vs skill will work.

Its an argument that will alway be simultaneously right and wring becayse its arguing how one fits a specific opinion over another while assuming their opinion is the only right opinion to be had.
 
There's probably some philosophical debate about negating the possibilities of amazings and terribles in service of averages, but I'd probably get too bored to pursue it for more than a lark.
It think it's more how it interfaces with the rest of the system. In many versions of D&D if the target is 12 say, then rolling 13 or 19 doesn't make much difference so the equal likelihood doesn't really matter.

In something like Traveller say where the difference between the target and what you roll does matter (say by adding to damage) then high rolls being equally likely makes the combat too swingy and ruins the flow of how it plays out.
 
I think the mechanic is O.K. I never really had a problem with modifiers, as long as there’s not like ten on each roll. If I were creating a game, I’d just use a flat +/-5 on d20 instead of a bunch of modifiers.
 
In many versions of D&D if the target is 12 say, then rolling 13 or 19 doesn't make much difference so the equal likelihood doesn't really matter.
Yeah, the obsession with the dice values some people have is another thing I don't understand. I'm ok with binary resolutions that don't require a different interpretation of every possible value of the die.
Obviously I couldn't get my head around FFG's Star Wars.
 
Modifiers in D20 become a metacurrency of their own. Its fun to play at lower levels, but like a lot about the system, it doesnt hold up as well in late game.
Its why I like the advantage ststem, it nerfs the metagame to near oblivion and shifts the focus back to pkaying the system instead of gaming it.
You have less desire to do increasingly unrealistic combinations to srack modifiers when advantages don't stack.

Sorry for the typos... small phone snd Im a giant....
 
Modifiers in D20 become a metacurrency of their own. Its fun to play at lower levels, but like a lot about the system, it doesnt hold up as well in late game.
Its why I like the advantage ststem, it nerfs the metagame to near oblivion and shifts the focus back to pkaying the system instead of gaming it.
You have less desire to do increasingly unrealistic combinations to srack modifiers when advantages don't stack.

Sorry for the typos... small phone snd Im a giant....
You've not run into people metagaming advantage?

And in this instance realizing that giving your opponent disadvantage can be a bigger bonus than getting advantage. That's the late game metagame. Advantage is worthless.
 
You've not run into people metagaming advantage?

And in this instance realizing that giving your opponent disadvantage can be a bigger bonus than getting advantage. That's the late game metagame. Advantage is worthless.
I don't think Adv / Disadv is that gameable a system because, outside of specific abilities, players can't easily get it; there isn't a defined "this gives advantage" list for them to metagame around. Now, players may ask for it, but at that point it's in the GM's control.

I'm also not that fussed about the possibility of the more skilled character losing to the less skilled character; it's unlikely, but it also keeps things interesting and exciting for players because it means sometimes they'll get the lucky break too. D&D isn't purely deterministic, and 5e's structure encourages players to be proactive because, sure, you've always got a fair-seeming chance.
 
I'm gming a low level 5e game now. Adv/Dis is working nicely so far.

Last night the archer was going to happily plink away from maximum range with disadvantage until he rolled a 4 & 20. That killed it for him and he moved up asap to get into normal range.

Conversely our damn cleric (friends since grade school) keeps getting target by disadvantage conditions and hasn't hit things in weeks.
 
I've never understood the obsession some people have with bell curves for resolution rolls. I've seen it recommended changing the attack roll in D&D to a 3d6 instead of a d20. Bizarre.
Anyway, not a fan of advantage/disadvantage myself. Mostly what I want from rules is sensible and easily understood calculations of probabilities. The more obfuscation there is, the harder it is to make appropriate decisions.
Well, in the case of Cold Iron's use of the normal distribution, it is used to have open ended rolls that follow a smooth curve rather than the "if you roll max, roll again and add" that creates a different sort of curve. Open ended rolls pretty much HAVE to be on some kind of bell shaped curve because you can't split an infinite number of results into the same fixed probability for each (you can't roll 1Dinfinity...). Of course, while one reason I like Cold Iron's use of the normal distribution is because it really is using the "true" bell curve, I also like it because of the clever way of generating points on the curve. Because one particular form of the normal distribution is to take a point on the curve, and calculate the area under the curve from minus infinity to that point for a curve where the total area under the curve is ONE, and what you want as an output is the value of X which is somewhere between minus infinity and positive infinity, you generate a random number between 0 and 1, and then figure out X (by looking at a table). So a bunch of d10s (0-9) are rolled to fill in each decimal place of that number between 0 and 1. Now because of the way the curve falls off from the central peak, you don't need to roll the infinite number of digits of that number from 0 to 1, you just need enough precision to make it mostly close. In fact, it turns out that 2 relevant digits of precision is good enough. That means near the center of the chart, you are rolling 2d10 to generate a number between 0.10 and 0.89, so you always start by rolling two dice (looking like D100, but really it's D100/100). Now if that number is actually between 0.00 and 0.09, you roll enough dice that you have two non-zero decimal places beyond a leading string of 0s, and if you roll between 0.90 and 0.99 you roll additional dice until you have two non-9 decimal places beyond a leading string of 9s. Now that you have your random number between 0 and 1, you look it up on a table and get a modifier between -infinity and +infinity, really between -40 and +40 on the table with the most precision we ever used in play. And the mathematical description of all of this sounds way more horrid that it is to actually use in play. In play, just roll two d10 like your rolling d100 and then resolve leading 0s or 9s, and look up on the table. You don't need to know the math at all... Oh, and because the 76% middle of the table results in a modifier between -8 and +8 you only need to memorize 17 numbers to be able to resolve 76% of all rolls without a lookup (and because of the nature of the mechanics and really bad rolls, you don't actually need to memorize all 17, and the bad rolls that fall below -8 usually are just a simple failure, you usually don't have to look up "I just rolled 0.0013, what the heck is that?" But the flip side is that when you roll a 0.9987 things are probably about to get exciting so it's really fun to do the lookup. So the resolution system actually flows pretty fast and creates a nice tension.

Now is it worth using that system versus many other systems? I dunno. Cold Iron is fun to play. So is L5R with it's exploding dice. So is Burning Wheel with it's exploding dice (that you usually have to spend meta-currency to explode. I'm assuming the advantage/disadvantage system provides it's own excitement, especially if you roll the 2nd die after rolling the 1st. Is advantage going to save you from the 1 you rolled first? Is disadvantage going to steal success from you? These mechanics aren't JUST about creating a particular pattern of probabilities, they are also about creating tension and excitement.
 
It think it's more how it interfaces with the rest of the system. In many versions of D&D if the target is 12 say, then rolling 13 or 19 doesn't make much difference so the equal likelihood doesn't really matter.

In something like Traveller say where the difference between the target and what you roll does matter (say by adding to damage) then high rolls being equally likely makes the combat too swingy and ruins the flow of how it plays out.
Which Traveller? :-) I play 1977 Traveller where margin of success is never used.
 
Which Traveller? :-) I play 1977 Traveller where margin of success is never used.
Quite right. I should have said Mongoose Traveller and derivative systems, e.g. Cepheus Light.
 
You've not run into people metagaming advantage?

And in this instance realizing that giving your opponent disadvantage can be a bigger bonus than getting advantage. That's the late game metagame. Advantage is worthless.
Gaming the advantages means participation. Gaming modifiers means mastery of the crunch, with most done in silence so the DM raises no eyebrows.

I much prefer advantages in every way in this regard.
 
KrakaJak KrakaJak get your pov. A few years back I I prob would have joined in on your side. My gaming evolved differently over time and now... with the people Ive played with recently this system is kismet. Ot simply works better than predecessors to encourage engagement and fun.

Is it mathematically better? For you, obviously not. Thats not the only merit to it though.

With all that, I can never agree its trash. Saying so repeatedly is just calling out proponents of it for badwrongfun.
 
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Gaming the advantages means participation.
I'm glad you enjoy your games. I don't not have fun when I play D&D 5e, but Advantage/Disadvantage was a highly praised mechanic that never felt good to me in play. Looking at the math, I'm glad I now understand why.

What I was disagreeing with is that advantage/disadvantage is not part of the crunch. It is.

Unless you are playing a game of mother may I with advantage for your players, there are rules for when advantage and disadvantage are given. Their are spells and feats that grant advantage, or give disadvantage (or grant conditions causing advantage/disadvantage). Advantage/Disadvantage IS part of the crunch. All down the line, once your bonuses are sufficiently high enough, Advantage is less valuable, and disadvantage more.
It gives the system a bunch of gotchas. Petrify is less effective than invisibility/darkness, charm is less effective than fear. Happens around level 5.

Before I read this, I felt it. As a character who buffed my party, tried to grant advantage is a myriad of different ways....I just was not as effective as curses or Bardic Inspiration. The most effective use of advantage is to counter disadvantage.

It's the reroll that causes the issue, not the simplification of (some) bonuses. If advantage/disadvantage was a simple +3/-3 bonus, the problems above go away.
 
I'm glad you enjoy your games. I don't not have fun when I play D&D 5e, but Advantage/Disadvantage was a highly praised mechanic that never felt good to me in play. Looking at the math, I'm glad I now understand why.

What I was disagreeing with is that advantage/disadvantage is not part of the crunch. It is.

Unless you are playing a game of mother may I with advantage for your players, there are rules for when advantage and disadvantage are given. Their are spells and feats that grant advantage, or give disadvantage (or grant conditions causing advantage/disadvantage). Advantage/Disadvantage IS part of the crunch. All down the line, once your bonuses are sufficiently high enough, Advantage is less valuable, and disadvantage more.
It gives the system a bunch of gotchas. Petrify is less effective than invisibility/darkness, charm is less effective than fear. Happens around level 5.

Before I read this, I felt it. As a character who buffed my party, tried to grant advantage is a myriad of different ways....I just was not as effective as curses or Bardic Inspiration. The most effective use of advantage is to counter disadvantage.

It's the reroll that causes the issue, not the simplification of (some) bonuses. If advantage/disadvantage was a simple +3/-3 bonus, the problems above go away.
Im not disagreeing that it is part of the crunch or it can't be gamed... It just reduces my headache to metagaming ratio greatly. I feel it to be an elegant solution to a problem I never fully realized existed before.
It really does comedown to preferences. Correct me if I am wrong, but It seems to me that you like a more math mastery style of game where things are more or less predictable and comfortable along a curve. That's cool, you take a technical approach to the game and I have gamed with many gamers like that before... in my younger years I was a lot like that too and would be still if I still played with m old gaming group.
Now, I like the more organic styles of play, myself. I do enjoy high crunch games that require mastery, for sure, but for me it is more that the system works well and there is options for me to develop my character along than the system working unerringly according to the math.
In the greater scheme of things, the way that the advantage systems works with the overall system makes it one of the better D&D rulesets I've played.The only thing preventing me from declaring it my favorite is that I feel too hamstrung and hindered by the rigid character development ark.I get very bored of my character when i can't do anything meaningful with it to change it. We all have our thing though.
 
So, do you guys like the way it's handled in Shadow of the Demon Lord where you add IIRC up to three d6's to a roll and either add (boons) or subtract (banes) them from the result on de d20? Boons and banes cancel eachother out.
 
I much prefer the Shadow of a Demon Lord variation of Boons and Banes.

You add or subtract a d6 from the roll instead and they can stack but you only ever take the highest score on the D6.

I find it to be less elegant in theory but to work better in practice. Allowing some degree of stacking means a bit more tactical thinking is possible and it can interface a lot more with the rest of the system. (This means that while the mechanic as a whole is slightly more complicated the system as a whole is simpler).

Advantage/Disadvantage seems a lot more elegant than it really is, because of all the ways it doesn't interface with other parts of the system.
 
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Overall players get advantage/disadvantage in a way that I never seen with any system of bonuses. Prior to its introduction in D&D 5e, I used a system of -4, -2, +0, +2, +4 bonuses in my Majestic Fantasy rules for a number of years. Then I started using advantage/disadvantage in its place. The difference was noticeable between the two in terms of how most players responded and utilized it. While the downside was the loss of the +/- 2 level of bonuses I granted before. However the advantage is that the maximum number one could roll was capped which I felt was a good point in its favor. I felt that positives out weight that disadvantage.

Irregardless of the math, its comprehensibility is off the chart.

And most players doesn't mean all players. In every group I found more than a few that were meh about the mechanic.
 
In play we've always seen Advantage not as a big bonus, but as insurance against shitty rolls. You don't usually use it for something you have a low chance of succeeding, you use it when you have a low chance of failure, but where failure will SUCK horribly.
 
I love advantage/disadvantage. It gives situation advantage without being super fiddlly, and doesn't change the range of possible results.

Bonuses too often get into "how can I get so many modifiers I can't lose" territory, which bores me.
 
That's not my experience at all.

Let's look at +5 athletics, vs +0.



You are a faster runner, and you have a 10 mph wind is at your back during a time trial, I should have no chance to beat you barring some supremely bad luck on your part (a misstep twisting your ankle etc.). You are more competent, and you have the advantage...you should destroy me here. You having advantage, I still have a ~15% chance to beat (or tie) you, down from 25% all things being even.

Now I have a 10 mph tailwind....same advantage. My chance of beating you is now ~50%. The same advantage that gave you a 10% more likelihood of beating me, has doubled my chances to beat you and completely evened the odds.

Now +5 vs +5 athletics. Equally competent.

We both have a 50/50 chance of winning. You get a 10 mph tailwind, advantage. You have a 75% chance of beating me. Advantage has made you more likely to beat a trained runner, than it did an untrained runner. Advantage and Disadvantage are dumb.
In an opposed d20 roll, as in this example, flat modifiers would pretty much amount to the same thing. Given that opposed rolls basically amount to d20-d20, adding +X would help the less trained party more than the better trained one.
 
I like it because it is simple and feels cool. I'm terrible at math so can't really comment there I just like mechanics that use dice, are easy to remember and speed up play.

Advantage/disadvantage seems to come from Jonathan Tweet btw, who introduced it in Over the Edge.
 
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