If you're hoping to evolve your Eevee into an Espeon or Umbreon in Pokémon Go Gen 2 there are a couple of different ways to do it. You can use the naming trick, but that only works once — or you can use a buddy system trick that's timed to the hour of the day.
Of course, you could also just evolve your Eevee like usual and hope for the best, but a recent study from the Silph Road found that the chances getting an Espeon or Umbreon this way in Pokémon Go are pretty much zero.
How to evolve Eevee in Pokémon Go: Espeon and Umbreon odds without any tricks
Silph Road researcher Kevlar2351 tested Eevee evolutions in a controller study. They evolved 45 Eevees, all at night and all without walking the Pokémon using the buddy system trick. The results yielded 24 Flareon, nine Jolteon, 12 Vaporeon, zero Umbreon and zero Espeon.
This seems like pretty conclusive proof that you need to use one of the two new tricks to get a Gen 2 Eevee evolution.
"I think this pretty much confirms for me that yes, you need to walk your Eevee at least 10 km (and gain two candies) in order to evolve it to either Espeon or Umbreon," wrote Kevlar2351. "I know this was mostly assumed by most everyone on the board here, but I hope this helps reassure that you MUST use the buddy system for either Espeon or Umbreon."
How to evolve Eevee in Pokémon Go: Does location matter?
Kevlar2351 also notes that their location, a desert biome in Denver, makes Flareon the most likely evolution. So it's possible that in some regions an Umbreon or Espeon could be more likely instead.
However, based on the results of this study it seems that the chances of evolving a Gen 2 Pokémon from Eevee without an tricks is actually zero. So it's probably not worth the risk when there are reliable ways to get the evolution you want.
More Pokémon Go guides, tips and tricks
Be sure to check out Mic's guides on how to get stardust, how to determine how long it will take you to reach level 40, the kind of Pokémon you get from 10-kilometer eggs, how to create new PokéStops, how to maximize your chances of catching Pokémon and how PokéStops distribute Pokémon eggs. Also check out how to catch Gen 2 baby Pokémon, our analysis of post-balance update Chansey and Rhydon and everything you need to know about finding the long-awaited Pokémon Ditto.