One of the new mechanics in Dungeons & Dragons fifth edition is advantage and disadvantage. Advantage and disadvantage are used in many of the situations where third or fourth edition would have granted a +2 or -2 modifier on a roll. When a character has either advantage or disadvantage on a roll, they roll two d20s rather than a single die. If the character has advantage, then the result is the higher of the two dice. With disadvantage, the lower die is used as the result. I’ve seen some people equate advantage and disadvantage to a +3 or -3 modifier on a roll, but the mechanics are actually a lot more interesting than simple modifiers.
The first interesting thing about the rules for advantage and disadvantage is how they stack on a roll. No matter how many situations grant advantage on a roll, a player will only ever roll two dice. Also, a single situation granting disadvantage on a roll is enough to cancel out any number of situations that grant advantage on the roll, and vice-versa.
Advantage and disadvantage also enable other effects to easily use them as triggers. For example, a rogue’s sneak attack class feature is usable whenever the rogue has advantage on an attack. Those triggers incentivize effects that give either advantage or disadvantage much more than effects that would give simple modifiers. For example, even if a disadvantage effect leaves a rogue with a good chance of hitting, the benefit of negating sneak attack means that an effect isn’t wasted. I hope there are more triggers like this when the core rule books are released over the next few months.
Because you roll two dice and then take one of them, the effect is that you have a higher probability of rolling at one end of the 1-20 range and a lower probability of rolling at the other. This is very different from the effect that a modifier has even if the average rolls are about the same. For example, on an attack with advantage you’re average roll is only about 3 higher than a normal attack, but you’re almost twice as likely to roll a 20 and get a critical hit (9.75% rather than the normal 5%). With disadvantage, your chance of getting a critical hit drops to a tiny 0.25%!
The change in the shape of the distribution also means that the chance of rolling above a certain DC can change a lot with advantage and disadvantage. For example, in a situation where a character needs to roll a 15 to succeed, they have a 30% chance of success. A +3 bonus would increase their chance of success to 45%. Advantage on the other hand, increases it to 51%. On the other hand, a -3 penalty drops the chance of success to 15%, but disadvantage reduces it to only 9%. At the same time, advantage and disadvantage maintain the same range of results as an unmodified roll instead of shifting the range like modifiers.
What do you think of the advantage and disadvantage mechanics in the latest edition of Dungeons & Dragons?