If Individual Values represent a Pokemon’s natural talent, Effort Values (or EVs for short) reflect the hard work and training put in by the Pokemon.
In a similar manner to IVs, each Pokemon has an EV in each of the six key stats: HP, Attack, Defense, Special Attack, Special Defense and Speed. To begin with, EVs are set to 0 in all stats (although there are exceptions).
As your Pokemon battles and defeats other Pokemon, that Pokemon will steadily gain EVs depending on the species of Pokemon defeated. Weak Pokemon generally award 1 EV in a given stat, while stronger ones can award 2 or even 3 EVs.
EVs can be gained at any time, even if a Pokemon is Level 100. However, EVs cannot be endlessly gained. Pokemon can gain a maximum of 252 EVs in a single stat and, on top of that, each Pokemon is restricted to a total of 510 EVs spread between all stats.
Like IVs, the term "EVs" is never used by the game; it’s just what fans used back in the days when there was no official term. As for the official term, it’s "base stats", which… is a very poor choice if we must be honest. By saying "base stats", you kind of imply the stats never change, but they do. Plus fans already use "base stats" for referring to a species’s stats.
The EV Advantage
While a Pokemon’s IVs are predominantly fixed, it should be clear that EVs can be customised. Although, as with IVs, you don’t need to concern yourself with EVs during the main story, as their influence is hardly felt. Only for tougher battles, should you start to consider planning.
At Level 100, a Pokemon gains 1 stat point for every 4 EVs accumulated (while at lower levels, the gain is proportionally smaller). Since a Pokemon can gain up to 252 EVs in one stat, that equates to a (you can do the Maths) 63 maximum stat boost in a single stat.
Before, we said a difference of 31 stat points was a big deal, so this is a big, BIG deal. Not to mention you’re allowed 510 total EVs, so you can have a second stat boosted by 63 points and a third stat boosted by 1 point (which is tiny, but better than nothing).
Needless to say, comparing a run-of-the-mill Pokemon taken from the main story with a random Nature and IVs and unoptimised EVs with a hardcore trainer’s Pokemon with a carefully selected Nature, the best IVs and optimised EVs is like looking at night and day!
Alright, so we’ve hopefully sold you on EVs, but how exactly do you customise your Pokemon’s EVs? Is it even possible? Well, of course, otherwise we wouldn’t be here! The primary method of EV training–the term coined for training your Pokemon in such a way to promote specific EV gain–is very simple: via good ol’ battling, with wild Pokemon no less.
As mentioned, each Pokemon species awards 1 to 3 EVs in a given stat. Therefore, all you need to do is identify which species gives what EVs, then focus on defeating that species. By the way, EV training is best done after the main story. Before that, careful EV management is simply unfeasible due to the varied range of opponents you face.
Outside of battles, you can feed your Pokemon vitamins such as Calcium and Carbos, which boosts EVs by 10 points in the specified stat. Trouble is, they’re expensive and cannot be used if the EVs in a stat are 100 or more. Wing items such as Genius Wing are not bound by such restrictions, but they boost EVs by 1 point each, which is tiny!
Moving onto newer methods, Isle Evelup in Poke Pelago allows you to passively EV train up to 3 groups of 6 Pokemon while you go about other business or take a break from the game. Finally, you can use Festival Coins to buy drinks for your Pokemon in the Festival Plaza that instantly maximise the EVs in one stat. However you can only enjoy this luxury once per day.
Oops, I Messed Up…
It happens to us all. Occasionally, you may fudge the Maths slightly or do something that causes you to gain the wrong amount or type of EVs. All is not lost though, as there are certain berries that claim to "lower" a particular stat, which actually has the effect of reducing EVs in that stat by 10 points.
These berries can be gotten from the berry tree halfway along Route 10. Afterwards, you can grow these berries in large numbers on Poke Pelago’s Isle Aplenny. You know, just in case…