As you know, the stat modifiers for Attack, Defense, Speed, Special Attack and Special Defense are as follows:
If your modifier is +n, then your stat becomes (n+2) * Stat / 2.
If your modifier is -n, then your stat becomes 2 * Stat / (n+2).
Your modifier is always a number between -6 and +6.
Now, accuracy and evasion modifiers are similar, being numbers between -6 and +6, but they modify the stats differently!
For the accuracy modifier:
If your modifier is +n, then you hit (n+3) * 100 / 3 per cent of the time.
If your modifier is -n, then you hit 3 * 100 / (n+3) per cent of the time.
For the evasion modifier, this is basically reversed, of course:
If your modifier is +n, then you get hit 3 * 100 / (n+3) per cent of the time.
If your modifier is -n, then you gethit (n+3) * 100 / 3 per cent of the time.
Your modifier is a number between -6 and +6 here too.
I'm posting this since there seems to be a general misconception of how much is Double Team's evasion.
For example, a Pokemon that uses Double Team once has evasion modifier +1, so it gets hit 3 * 100 / (1+3) per cent of the time, or 75% of the time. If it uses Double Team again, it now gets hit 3 * 100 / (2+3) percent of the time, or 60%. If you use the same formulae as for the normal stat changes, you get 66.6% and 50%, which is wrong.
If your modifier is +n, then your stat becomes (n+2) * Stat / 2.
If your modifier is -n, then your stat becomes 2 * Stat / (n+2).
Your modifier is always a number between -6 and +6.
Now, accuracy and evasion modifiers are similar, being numbers between -6 and +6, but they modify the stats differently!
For the accuracy modifier:
If your modifier is +n, then you hit (n+3) * 100 / 3 per cent of the time.
If your modifier is -n, then you hit 3 * 100 / (n+3) per cent of the time.
For the evasion modifier, this is basically reversed, of course:
If your modifier is +n, then you get hit 3 * 100 / (n+3) per cent of the time.
If your modifier is -n, then you gethit (n+3) * 100 / 3 per cent of the time.
Your modifier is a number between -6 and +6 here too.
I'm posting this since there seems to be a general misconception of how much is Double Team's evasion.
For example, a Pokemon that uses Double Team once has evasion modifier +1, so it gets hit 3 * 100 / (1+3) per cent of the time, or 75% of the time. If it uses Double Team again, it now gets hit 3 * 100 / (2+3) percent of the time, or 60%. If you use the same formulae as for the normal stat changes, you get 66.6% and 50%, which is wrong.