Effort Value (EV) Guide
Okay, first let me start by saying: EV training isn't for everyone! It involves quantitative work not required to train your Pokémon. If you're looking to get the most out of your Pokémon's stats though, keep reading.
Effort Values (or EV points for short) are numeric points that go into each of your Pokémon's stats: HP, Attack, Defense, Special Attack, Special Defense, and Speed. A Pokémon can gain no more than 510 EV points, and the maximum amount of EV points that can go into a specific stat is 255. The idea is that you want to put an effective number of EV points into one or two stats to increase them to their maximum potential with minimal EV point waste. Although you could distribute your EV points evenly to each stat if you wanted to, however it would not make the most significant difference since there would be less EV points to go into each stat. Basically, the more EV points you put into one or two stats, the better.
There are certain EV points that you get from certain Pokémon in the games that increase certain stats. For example, a wild Whismur in Ruby/Sapphire/Emerald will yield you one HP effort value point.
Before you begin EV training, you need to write out how many EV points you want to put into each stat. Once you have figured that out, you should equip the "Macho Brace" item to that Pokémon. This item doubles the amount of EV points the Pokémon receives. So, defeating a Whismur would yield you two HP EV points instead of one. Furthermore, if by chance your Pokémon has the virus Pokérus, it will also double the amount of EV points received in battle. So, that combined with the Macho Brace would earn you 4 HP EV points from a Whismur. Quite a deal!
To see a list of Pokémon that yield EV points and for which stats, please visit the EV Pokémon Locations page.
Source:
1 Chu, Clio. "EV Training Spots"." Pichu's World. 2007. 8 Dec 2008 http://www.pichusworld.net/EVtrainingspotsDP.php.
