Evasion moves like Double Team essentially decrease the accuracy of all moves coming your way. There are 6 stages:
0 = 100%
1 = 75%
2 = 60%
3 = 50%
4 = 42.85%
5 = 37.5%
6 = 33%
So if you use Double Team once, and the opponent uses a move with 100% accuracy (e.g. Flamethrower) it will now have a 75% chance of hitting.
Some moves like Defog also decrease evasion, again this is 6 stages:
0 = 100%
-1 = 133%
-2 = 166%
-3 = 200%
-4 = 233%
-5 = 266%
-6 = 300%
So after 3 times lowering evasiveness you should have double the chance of hitting the opponent.
