Advantage probabilities

There are two types of advantage

For hit rolls and ability checks, you roll twice and take the higher result for advantage, or the lower result for disadvantage
For skill rolls, you roll an additional 3d6 for advantage, or you remove 3d6 for disadvantage

The following table shows the expected value when rolling a hit roll at disadvantage, normally, and at advantage

Dice Rolled	Disadvantage	Normal Roll	Advantage
3d6	1.0	1.5	2.0
4d6	1.6	2.0	2.5
5d6	1.9	2.5	3.1
6d6	2.3	3.0	3.7
7d6	2.8	3.5	4.2
8d6	3.2	4.0	4.8
9d6	3.7	4.5	5.3
10d6	4.1	5.0	5.9
11d6	4.6	5.5	6.4
12d6	5.0	6.0	7.0

The following table shows the expected value when rolling an ability check at disadvantage, normally, and at advantage

Dice Rolled	Disadvantage	Normal	Advantage
3d6	8.8	10.5	12.2
4d6	12.1	14.0	15.9
5d6	15.3	17.5	19.7
6d6	18.6	21.0	23.4

The expected value of adding 3d6 to a skill roll is roughly 0.5