Effort Values are a point system which affects the stat growth of an individual Pokémon. When a Pokémon battles and defeats another - either wild or owned by an NPC trainer - they earn Effort Points specific to the type of Pokémon defeated. For example, defeating a Kakuna awards one defense Effort Point, whereas defeating a Gastly awards one special attack Effort Point.
Understanding Effort Values
Effort Values are not visible in the official Pokémon games, but they can be viewed in PokeMMO on the third page of the Pokémon's status screen. In the official Pokémon games, each individual stat can have a maximum of 255 Effort Points, with the Pokémon able to have a total number of 510 Effort Points across all stats. All Effort Points in a particular stat are then divided by 4 and the quotient is added to the corresponding stat over the course of several levels.
With 252 - 255 Effort Points allocated to one stat, that stat will be increased by a total of 63. Because Effort Values are divided by four, the maximum number of Effort Points that should be trained in one stat is 252 - this being the number closest to 255 that is divisible by four. Furthermore, there will always be two Effort Points out of 510 left over, unable to affect any stats. It is for this reason that in PokeMMO, the maximum number of EVs in a stat has been reduced to 252 and the total number of EVs reduced to 508, to ensure that there can be no excess points left over.
To properly utilize Effort Values, many trainers practice EV Training. By selecting specific Pokémon as targets, trainers can choose the allocation of their Pokémon's Effort Points. Pokémon are generally EV trained to fill a specific role in their trainer's team - for example, a defensive 'wall' Pokémon will often be trained to have high HP, Defense or Special Defense EVs, and therefore will be much more resilient.
Reaching the limit of 508 total Effort Points can take a long time. Many EV trainers use a hold item called the Macho Brace, which doubles the amount of Effort Points earned by defeating opponents. The Macho Brace is found by using the Itemfinder on the spot Giovanni stood in the Viridian Gym, after he has been defeated.
EV Training Locations
There are a number of places in Kanto which streamline the task of acquiring desirable Effort Values.
HP Training Locations
The water in Viridian City has a 50% encounter rate of Slowpoke, which offer one HP EV when defeated. Alternatively, Caterpie in Viridian Forest also yield one HP EV.
Once you have unlocked the later Sevii Islands you will have access to several additional EV training locations. Dunsparce at the 3 Island Port give one HP EV and have a 100% encounter rate. Surfing in the 4 Island Pond will bring Psyduck (1 SP Atk EV), Wooper (1 HP EV), Marill (2 HP EVs) and Slowpoke. Surfing in the Ruin Valley Pond on 6 Island will bring Wooper and Marill.
Attack Training Locations
Paras, which give Attack Effort Points, are the sole Pokémon that can be encountered on B1F of Mt. Moon. This floor can be easily accessed from the Pewter side of Mt. Moon by using the first ladder seen on the main route through the tunnels.
Another option is to use the Super Rod in the "Warden's Pond" in Fuschia City. Goldeen (1 Atk EV), Seaking (2 Atk EVs) and Gyarados (2 Atk EVs) are all plentiful there. However with the implementation of the fishing animation this location has become less popular.
Defense Training Locations
Metapod (2 Def EVs) and Kakuna (1 Def EV) can both be encountered in the Viridian Forest.
Route 21 has a single patch of grass south of Pallet Town which requires Surf to access. Here, only Tangela can be encountered, which offers one Defense EV when defeated.
Special Attack Training Locations
Gastly and Haunter have a high encounter rate in Lavender Town's Pokémon Tower, both of which yield Special Attack EVs when defeated. This, combined with the purified healing zone located in the tower make this a popular place to train for Special Attack.
Another common location is B3F of Ruby Path (within Mt Ember). This location is 100% Slugma, giving 1 Sp Atk EV each and allowing the use of the bike while EV training.
Special Defense Training Locations
Tentacool are located in all ocean bodies of water. These include the Surf routes 19, 20 and 21 leading to and from Cinnabar Island.
Hoppip, with a 100% encounter rate on the 5 Island Memorial Pillar, also yield 1 Sp Def EV.
Speed Training Locations.
Route 1, during day time, has a 100% encounter rate of Rattata and Pidgey (1 Speed EV each).
Digglet's Tunnel is entirely Digglet and Dugtrio, 1 and 2 Speed EVs respectively.
The 4th cycle of the Altering Cave is 100% Zubat, giving 1 Speed EV
Using the Old Rod to fish Magikarp also yields 1 Speed EV