The EVs, or effort values, are like specialized training a pokemon has undergone. A pokemon has separate EVs for each of its six stats. When a pokemon is first caught or hatched, all its EVs are zero. Any single EV can go as high as 255, but the sum of all a pokemon's EVs cannot exceed 512. At level 100, a pokemon's stat increases by 1 for every 4 EVs it has earned; it is possible for a properly-trained pokemon to have two stats raised 63 points by EVs and a third by 1 point.
EVs are earned fastest from vitamins, but a vitamin may not raise an EV for a single stat over 100 (which is why an untrained pokemon can only eat 10 vitamins of a single kind, and a trained pokemon can eat even less). EVs are also earned through battle; any pokemon that receives experience also receives EVs. Each individual species of wild or trainer pokemon is worth its own set of EVs. For example, a Magikarp is worth 1 speed EV, and a Blissey is worth 3 HP EV.
This EV training through fighting can be monotonous, though there are a few methods to speed it up:
- Fighting low-level pokemon (as the level has no bearing on earned EVs)
- Infecting the pokemon you are training with Pokerus (which doubles earned EVs)
- Giving the pokemon you are training a Power item (which raises earned EVs in the specified stat by 4)
With all three methods above in effect, it is possible to max out a single stat's EVs in a relatively short time; if a pokemon that has or has had the pokerus defeats a Blissey while holding a Power Weight, it will gain 10 HP EVs...the same as an HP Up (3 x 2 = 6, and 6 + 4 = 10).
Feathers also raise an EV by one. They work like vitamins, though they are not limited to the 100 EV ceiling like vitamins.
Finally, there are a set of six berries that LOWER an EV by 10. They are like reverse vitamins.
Feathers and EV-lowering berries are primarily used for fine-tuning EVs, since an EV's effect maxes out at 252 (255 would be a waste of 3 EV points).