Gold star for effort
You may have heard hardcore poke-fans throwing around the term "Effort Values" when talking about training their pokes to the limit for competitive fighting. Effort Values are a hidden aspect of pokemon training that rewards your pokemon with extra stat boosts. It's not necessary to pay attention to (or even know about) Effort Values to beat the game. But if you aspire to be a super trainer for competitions and you want to build specific strengths of your Pokemon as much as possible, Effort Values will be crucial to your success.
If that last sentence describes you, read on. Otherwise, don't fret too much about it - this is just additional information for the most devoted trainers.
Effort values are special points that affect the stat gains your Pokemon earns in battle. You can't actually see effort values on your status screens, but they're there, lurking deep within the programming code. A single pokemon can earn up to 510 Effort Value points in total spread across 6 categories: HP, Attack, Defense, Special Attack, Special Defense, and Speed. For each 4 points it earns in a single category, it will gain 1 extra stat point in that area when it hits level 100. EVs also don't reset upon level-up - they accumulate throughout the game.
Effort Value points can be gained through two methods: vitamins and fighting. Vitamins provide a quick and easy EV gain: In addition to a 1-point stat boost in a particular area, vitamins will yield 10 EV points to that same statistic. You can only use 10 vitamins on a pokemon, though, so the largest amount of EVs you can gain in a single category using vitamins is capped at 100.
Above: Check the EVs on Dialga. And don't mess with him. He's got silver horns and stuff
Most of your EVs will probably be earned through fighting. Whenever a pokemon gains EXP in battle, it also gains EVs based on the enemy it defeated. Different enemy pokemon yield different EV values in different categories. Fighting a pokemon that has particularly strong stats in one area will yield more Effort Value points in a certain category than a more well-rounded Poke. For example, fighting a Golem will yield 3 EV points for defense, while Skorupi only gives you one. If you've got a Macho Bracer equipped, or if your Pokemon have contracted the rare Pokerus sickness from a random battle, Effort Value gains in battle will be doubled.
In total, you can gain 127 additional stat points for a single pokemon. No matter how you slice it, that's a pretty substantial amount of potential gain!
If for some reason you want to reduce some of your accumulated EV points, you can do it by feeding pokes certain berries. They lower EVs by 10 points each.
· Pomeg Berry: lowers HP effort value.
· Kelpsy Berry: lowers Attack effort value.
· Qualot Berry: lowers Defense effort value.
· Hondew Berry: lowers Special Attack effort value.
· Grepa Berry: lowers Special Defense effort value.
· Tamato Berry: lowers Speed effort value.
Unfortunately, since you can't see EVs, the only way to know how many EVs you've gained in each category is by using a chart of pokemon EV values, a pencil, paper, and a calculator from the moment you first put your poke into battle. We can at least provide the charts for you, but you're on your own for the rest.
