I recently evolved 2 of my Ratatta in Pokemon Go. First one was about 280 CP and it evolved into 555 CP Raticate, while other Rattata that was only about 200 CP still evolved into 552 CP Raticate. Is there are some other criteria other than CP to kept in mind while evolving pokemon?
There are two reasons to evolve Pokémons:
- To gain EXP.
- To get new, Powerful, Pokémons.
When you're simply trying to gain EXP the CP of the Pokémons does not really matter as you'll be transferring a lot of them. You should not be concerned about the CP at all. Don't forget to pop a lucky egg for double exp!
You should be concerned about how strong your Pokémons can get when you're evolving for the sake of getting as powerful Pokémons as possible. Essentially you're thinking about CP. EuroGamer describes CP in a pretty neat manner:
CP, or Combat Points, is a measure of how strong your Pokémon will be in battle against another Pokémon, and is actually a combination of several hidden stats. Each Pokémon in Pokémon Go has a set of Base Stats for Attack, Defence, and Stamina, along with a hidden Level and a hidden modifier for those Base Stats, known as a Pokémon's IVs.
What this means is that there's more to every Pokémon than it's CP. The base stats are relatively more important than CP because the IV determines how possibly strong in terms of CP the Pokémon can get once it's been powered up completely. This is only a matter of a few percents but for some it matters a lot.
If you're just having fun in the game and not too concerned about IV and how possibly strong your Pokémons can get you should simply be focusing on evolving your strongest Pokémons in terms of CP, as it's the easiest way to do it.
If you really want to get the very best you should be using an IV Calculator to calculate which of your Pokémons have a % chance of becoming close to being perfect and which ones are close to being "garbage" and should be transferred instantly. Note that move-sets are more important, though, but you want to get the best move-sets on the best Pokémons. It's kind of sad to get the best combo on a Pokémon that will only having e.g. 50% IV if you're a player who aims higher.
A 400 CP Rattata could become a weaker Raticate than a 350 Rattatta's evolve if the latter's IV was so much higher than the previous one, after powering both to the maximum. E.g. 1150 vs 1175 when both are maxed out.
It's important to look at the maximum and average %, preferably you want all numbers to be the same as you'll be 100% sure of how good your Pokémon truly is.
If you cannot be arsed to IV calculate everyone you can try to find a balance between finding the best combination of CP, HP (Health-Point) and lowest powering up cost, the greater the two are and the smaller the last the more likely it is to have good IV.
I strongly recommend saving (e.g. by renaming your Pókémons and add an e at the end of the name) hatched Pokémons as they're more likely to have higher IV than usually. Those are the ones you'll most likely be wishing to focus on evolving and powering up into beasts.
Last but not least: It's important to know that at level 20 you stop getting "better" Pokémons from Eggs and at level 30 you stop getting "better" Pokémons in the wild. I.e. everyone at that level and above have the same chance of getting equally good Pokémons but of course the number of power ups are limited to your trainer level. I'll add a source for this if I manage to dig it up.
If you evolve purely to gain experience it doesn't matter what the pokemon stats are (using lucky eggs while evolving many pokemon will help you gain even more xp).
If you aim to eventually get the best possible pokemon of that kind, lets say you want to have the best Raticate possible, you want it to have as good Individual Values (IVs) that compose of your pokemons attack, defence and stamina, that aren't visible in the app, but using an IV rater you can see how good these values are (just google 'pokemon go IV rater'). Also better raters show all possible attacks that a certain pokemon can have (usually values of power/second given). You want your best pokemon to have the best attacks (this might be even more important than IVs). Also if your pokemons attacks normal(fast) and special compose from different kinds (Steel, Water, Fire etc.) they might be more diversely better against other types. So if your pokemon attacks only in with Fire attacks and you encounter a Water pokemon that resists your attacks then you might wish that either the normal or the special attack would be of another kind.
My personal tactic is that regarding a pokemon family (e.g. Charmander, Charmeleon, Charizard) I keep pokemon with good CP until I catch, train or evolve a better one. I never send a pokemon to the professor unless I have the same pokemon or a pokemon from the same family that has better combined IV + attack set. Also just to feed my inner (but minor) OCD I tend to keep one Charmander and Charmeleon even if I have a good Charizard measured in CP and IV.