In Pokémon Go, Individual Values (IVs) are hidden stat modifiers that can range from 0 - 15. Each Pokémon has three IVs (Attack, Defense & Stamina).Attack: Determines damage dealt.
Defense: Determines damage taken.
Stamina: Determines HP.
IVs are added to a Pokémon's base stats before calculating CP. This means that Pokémon of the same species with higher IV will have greater CP than those with lower IV. IVs are random and can vary among Pokémon of the same species.
Take Blastoise as an example. Its base stats are as follows:
Blastoise Base Stats
Attack: 186
Defense: 222
Stamina: 158
However, if this Blastoise had perfect IVs, its stats would be as follows:
Blastoise Base Stats (with IV of 15/15/15)
Attack: 201
Defense: 237
Stamina: 173
As you can see, IV can add up to about a 10% increase in Attack, Defense or Stamina (depending on the Pokémon's base stats).
While IV does not have significant impact on a Pokémon's overall power, it does allow for slightly higher max CP. In addition, a Pokémon with perfect IVs will deal approximately 4% more damage.
Some players focus on Powering Up their highest IV Pokémon, while others focus on Powering up their Pokémon with the highest DPS Moves. In a perfect world you will want to Power Up Pokémon with ideal movesets and perfect IVs.
Note: Use the IV Calculator from Pokeassistant.com to determine the IV of your own individual Pokémon.
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