There are 2 ways of reporting these statistics. The majority of people in the past few pages are doing it the slightly more misleading way. If you have to roll an 11, Advantage will change your chances from a 50% to a 75% chance. You can technically call this a +25% boost. However, this runs into problems when you run into more extreme die values.
The better way to state this is: Your chances have gone up by 50%. Stating it this way-as a ratio involving the old and new percentages-is more intuitive.
Similarly, if you need a natural 20 to succeed, then Advantage doubles your chance of success. (@SacredWitness: You'd say that this is only +5%) --if you need a natural 6, Advantage makes you 25% more likely to succeed --if you need a natural 16, Advantage makes you 75% more likely to succeed. (What's the average of 25% and 75%?)
Thus, on average, Advantage makes you 50% more likely to succeed. This can be stated, for simplicity, as a +5 bonus on the die because 5e is based on bounded accuracy: usually you'll need around a 10 to hit
p.s. You also have to take into account the fact that natural 1s are misses and natural 20s are auto hits (for double damage!), which make Advantage even better by massively increasing your chances of a crit.
Last edited by mrfuji3; 13/11/2004:35 AM. Reason: natural 5 to natural 6, natural 15 to natural 16