Guides
Effort Values
Effort values, or EVs, are hidden numbers that boost growth of each stat. They are accumulated much like experience points; however, unlike experience points, they are entirely hidden. Through winning battles and other means, Pokémon may accumulate EVs for each stat. It is the earning of these points that differentiates a "wild" Pokémon from a "trained" one.
Accumulating
Different Pokémon give different EVs when they are defeated. Typically, the EVs are related to the best stat(s) of the Pokémon in question. As an example, Chansey, with its superb HP stat, naturally gives HP EVs.
As with experience points, only so many EVs can be accumulated. A Pokémon may earn a maximum of 510 EVs, with the maximum for any given stat being 255. As there are six stats, and 6 × 255 = 1530, which is way more than the maximum of 510, there is a lot of room for planning what EVs will be given. Therefore, it is up to the trainer to strategically accumulate EVs in a way that is most beneficial to that particular Pokémon's growth.
To make deciding what Pokémon you wish to battle easier, have a look at the list of Pokémon and their EVs.
Easing the process
Effort value training could possibly be more tedious than regular training for experience points. While there are nowhere near as many EVs to earn as there are experience points, it is necessary to defeat only the Pokémon that give the desired EVs. Thankfully, there are a number of ways to speed up the process. Combining them all, effort value training can be less of a hassle.
Vitamins
These items have been around since even the first generation of Pokémon. Expensive as they may be, each one adds 10 EVs to the relevant stat. Unfortunately, only 10 of each type may be given to a Pokémon, so unless equally distributed EVs are desired, it is likely that training will still be required.
HP Up- Adds 10 HP EVs.
Protein- Adds 10 Attack EVs.
Iron- Adds 10 Defense EVs.
Carbos- Adds 10 Speed EVs.
Calcium- Adds 10 Special Attack EVs.
Zinc- Adds 10 Special Defense EVs.
Held items
There are numerous held items in Pokémon Diamond and Pearl that pertain to EVs. However, all of these items do have the side effect of halving the holder's speed, so training with them can be considerably more difficult. Fear not; speed will return to normal when the item is removed.
Macho Brace- While a Pokémon is holding this item, it earns twice as many EVs as it would have otherwise earned. This item can be acquired in Pastoria City by showing a man all three of Burmy's forms in party.
Power Weight- Adds 4 HP EVs after winning each battle. Costs 16 Battle Points at the Battle Tower.
Power Bracer- Adds 4 HP EVs after winning each battle. Costs 16 Battle Points at the Battle Tower.
Power Belt- Adds 4 Defense EVs after winning each battle. Costs 16 Battle Points at the Battle Tower.
Power Anklet- Adds 4 Speed EVs after winning each battle. Costs 16 Battle Points at the Battle Tower.
Power Lens- Adds 4 Special Attack EVs after winning each battle. Costs 16 Battle Points at the Battle Tower.
Power Band- Adds 4 Special Defense EVs after winning each battle. Costs 16 Battle Points at the Battle Tower.
PokéRus
PokéRus, the exceedingly rare virus first introduced in Gold and Silver, now doubles the acquired EVs of any Pokémon that is or was infected by it. This doubling occurs after the effects of the items listed above; for example any Pokémon with PokéRus holding the Power Bracer would always get at least 8 HP EVs per battle!
Thanks to the online features of Pokémon Diamond and Pearl, getting PokéRus is now relatively easy. Ask your friends if they have it, and if they do, get them to help you infect one of your Pokémon. That Pokémon can then transmit it on to the rest. But remember that PokéRus goes away (the Pokémon, though it still gains double EVs, can no longer transmit it) in a few days unless the infected Pokémon is stored in the PC, so take care not to lose it.
Removing EVs
What happens if unwanted EVs are acquired? Thankfully, it is possible to remove EVs through the use of berries. These berries all raise a Pokémon's happiness while removing EVs from a single stat.
Pomeg Berry- Removes HP EVs.
Kelpsy Berry- Removes Attack EVs.
Qualot Berry- Removes Defense EVs.
Tomato Berry- Removes Speed EVs.
Hondew Berry- Removes Special Attack EVs.
Grepa Berry- Removes Special Defense EVs.
When to train
With all of that taken into consideration, when is the appropriate time to acquire the desired EVs? Realistically, because of the berries that can remove unwanted EVs, they can be acquired at any time. However, it is desirable that EV training be completed before level 100, because at that point it is no longer possible to level up, so only the use of vitamins will recalculate a stat.