D&D 5E Break this House Rule: Advantage(s) can stack

[Esker]

In case anyone is interested, here's a colorful picture showing how additional dice affect the chances of getting a particular roll.

So we can see that the more dice we roll, the more the distribution of outcomes clusters near the high end.

And here's one showing the chances of success when you need a particular natural roll or better to succeed.

And here's one showing the effect of the additional dice converted to an equivalent bonus (i.e., +1 corresponds to an increase in success chance of 5 percentage points, +2 to 10 percentage points, etc.)

As most people are probably aware, "regular" advantage is the most valuable when the baseline chance of succeeding is near 50%; in other words when you need to roll a natural 11 or better. It's worth up to +5 in this case. When we roll three dice, the cumulative benefit compared to rolling once is the greatest for moderately difficult rolls; when we need a 13 or better to succeed. In this case, having advantage is equivalent to a bonus of about +7.5. We can see for any particular target number the added value of each additional die is going down; but also the zone where advantage is worth the most is moved to the right, namely to more and more difficult rolls. If we get to roll 8 dice, the value of advantage would go all the way up to an equivalent bonus of +13, when the natural roll needed to succeed is 16... but three dice would be worth +6.5 in this case, so the extra five are combined only worth as much as the second and third.

Where going beyond three dice really helps us is for really difficult rolls (like getting crits): each additional die is still worth a little less than the previous one, but we have fewer and fewer "multiple successes" as we go up in difficulty, to the point that getting to roll eight dice raises our crit chance almost sevenfold.

 

[FrogReaver]

View attachment 107505

you might wanna rename that one...

 

[Esker]

you might wanna rename that one...

Hah, yeah... I figured out how to embed images in the post, so edited it to do that instead.

 

[FrogReaver]

The danger here I think is more in party synergy than in individual PC's. Assuming a party with a PAM + GWM fighter or barbarian you could potentially give the GWM 2-3 sources of advantage which really ramps up their damage output against most foes.

 

[Gladius Legis]

I think the only context advantage and disadvantage instances should "stack" should be determining whether advantage vs. disadvantage cancels out. More sources of advantage than disadvantage = advantage. Equal number = neither. More disadvantage sources = disadvantage.

 

[Esker]

One consequence of the skewed distribution of die results that happens when we have advantage is that mechanisms like precision attack (or bardic inspiration) that let us add a die roll to a d20 roll become better, since when the distribution is skewed, misses become more and more likely to be near misses, and therefore we will more often have outcomes that these extra dice can convert to successes. If we get to add a d10 to our roll, for example, we will automatically convert a failure-by-1 to a success; we have a 9/10 chance of converting a failure-by-2, etc. With advantage, failure-by-1 is more likely than failure-by-2, which is more likely than failure-by-3, etc.

On average, if we have a d10 in our pocket that we can add to a roll when we fall short (as a Lvl 11 Battlemaster with precision attack), here's the combined chance that we wind up with a success that would have otherwise been a failure:

Factoring that into the baseline chance of success, here's the bottom line success rate for different target rolls (before any modifiers), and different numbers of d20s, assuming we get to add a d10 to our roll

So, going back to the example where we needed a natural 15 or better to hit an enemy, we're already at a 90% chance of hitting with elven accuracy and precision attack; extra dice can't do that much more. So for our hypothetical all-out first round nova assassin/paladin/fighter, if we're not increasing our chance to crit, it's far better to invest in more damage dice and more attacks than to pour resources into chasing ever-diminishing returns of stacking advantage on top of advantage.

 

[Esker]

The danger here I think is more in party synergy than in individual PC's. Assuming a party with a PAM + GWM fighter or barbarian you could potentially give the GWM 2-3 sources of advantage which really ramps up their damage output against most foes.

I don't know, I mean, even for difficult rolls (when it has the biggest impact), adding a second source of advantage is only boosting the barbarian's chance to hit by something like 15%. If the other party member could do even 1/3 the damage that the barbarian does on a hit and has say a 50% chance of hitting, they're better off trying to do that damage themselves than using their action to contribute that second source of advantage. (I realize that not all ways of granting advantage require that the ally forego entirely their own ability to do damage, so things like familiar help could still be an issue, but I question whether things like shoving prone or devoting your concentration to a spell like faerie fire are often going to be worth the action/concentration cost for a third die)

 

[FrogReaver]

I don't know, I mean, even for difficult rolls (when it has the biggest impact), adding a second source of advantage is only boosting the barbarian's chance to hit by something like 15%. If the other party member could do even 1/3 the damage that the barbarian does on a hit and has say a 50% chance of hitting, they're better off trying to do that damage themselves than using their action to contribute that second source of advantage. (I realize that not all ways of granting advantage require that the ally forego entirely their own ability to do damage, so things like familiar help could still be an issue, but I question whether things like shoving prone or devoting your concentration to a spell like faerie fire are often going to be worth the action/concentration cost for a third die)

Often times such characters will already be using such abilities.

Faerie Fire.
Entangle
Web
Hold Person
Hold Monster
Open Fist Monk using flurry of blows
Battlemaster using trip attack
A greater invisibility buff

etc. Many sources of advantage are already abilities that are good to use and already are used even without the proposed stacking effects.

 

[MarkB]

As most people are probably aware, "regular" advantage is the most valuable when the baseline chance of succeeding is near 50%; in other words when you need to roll a natural 11 or better. It's worth up to +5 in this case. When we roll three dice, the cumulative benefit compared to rolling once is the greatest for moderately difficult rolls; when we need a 13 or better to succeed. In this case, having advantage is equivalent to a bonus of about +7.5. We can see for any particular target number the added value of each additional die is going down; but also the zone where advantage is worth the most is moved to the right, namely to more and more difficult rolls. If we get to roll 8 dice, the value of advantage would go all the way up to an equivalent bonus of +13, when the natural roll needed to succeed is 16... but three dice would be worth +6.5 in this case, so the extra five are combined only worth as much as the second and third.

And presumably, the reverse applies for cumulative Disadvantage, with higher number of dice rolled tanking your chances of meeting even relatively easy DCs.

 

[Esker]

And presumably, the reverse applies for cumulative Disadvantage, with higher number of dice rolled tanking your chances of meeting even relatively easy DCs.

Yeah, you can get the graphs for disadvantage by reversing both the x and y axis: label the targets 20 to 1 going left to right, and success probabilities from 1 to 0 going bottom to top.