[tennisace]

To add onto iss's answer, if you are using a Hidden Power which reduces a key stat (HP Fire/Ground/Fighting generally), you need to add 4 EVs to your EV total for that stat to get the full boost in "actual stat". To illustrate, say you have a Gastly with HP Fighting. The IVs for Speed and Special Attack are reduced to 30. Normally you would use 236 EVs in each to maximize them, however, you would have to use 240 EVs in each because of the IV drop.