pokemon-go
As some users have experienced, there is potential to receive a candy of the type corresponding to a Pokémon you feed in any of your team's gyms. This is especially useful to players who haven't been able to amass enough candy to evolve their Chansey or Dratini yet.
I have yet to personally get any candy this way, though, so this is clearly not a guarantee. What is the chance that I get a candy as a bonus for feeding a Pokémon in a gym?
It seems that the way it preselects these 6 is by finding the best type matches for the opponent. From what I've seen it tends to base this on typing and movesets, then sorts by CP and HP.
That said, I haven't seen it suggest what I would call the "optimal" 6 Pokemon for the entire fight. It looks like it selects all 6 to combat a single opposing Pokemon, typically the first one you will fight. It seems like it merely chooses an advantageous type matchup Pokemon with the highest CP then goes down the list.
I was able to find some evidence to support what I've said here and here.
tl;dr: While no concrete numbers have been confirmed yet, the odds of finding one from a PokéStop are very slim. Based on the reports I've been reading, it looks like the odds to receive one are at or around 1%, possibly lower. In fact, from this report, the calculated probability of receiving an evolutionary item seems to be roughly 0.14%, as a grand total of 33 evolutionary items were received among over 24,000 drops.
However, it's worth noting that it seems like you are now guaranteed to get an evolutionary item when spinning your 7th day streak PokéStop, according to this in-game notification:
Until this update has been around for longer, it's not possible to have the most accurate numbers in terms of the probability of these items appearing from PokéStops, but numerous users have been reporting that they received an evolutionary item as a daily streak bonus reward (especially so for 7th day Streak bonuses).
For instance, this thread:
Probably coincidence, but I got my first and only evolution item from my 7-day streak Pokestop spin. 100+ spins before that came up empty.
I got one last night. But also got one on my 7-day streak this morning.
Got a metal coat from 7 day spin
One comment linked to this thread which originally speculated that these items only came from 7th day streak spins (this has since been disproven numerous times, though it has now been confirmed that you are guaranteed an evolutionary item for this):
I got my Sun Stone on my 7 day spin this morning too. Havent found an evo item since. Increased chance on 7 day streak spins? Or just plain luck?
I got an upgrade with my 7 day streak this morning. It may be that we're guaranteed an evolution item at each 7 day streak but you can also get them randomly at pokestops.
I also got my sunstone today at my 7th day streak. My friend got a dragonscale in his first spin of the day( normal streak).
7th day spin for me this morning too, I got an Up-Grade
I just spun for my 7th day streak - got Upgrade. Prior to that, I had to spin pokestops for almost 8 hours before I received a King's Rock.
As many have noted, however, you are able to get these items randomly from PokéStops. Some users in that thread also estimated how many spins it took them to find their first evolutionary item. From that thread:
I spun over 400 yesterday to get my first sun stone. this took 5 hours of downtown grinding.
I spun over 100 today as well as yesterday and nothing so far. Not even for my 6 day spin.
About 250 spins today, got a Sun Stone from a random spin.
Meanwhile I'm on spin 175 for today and still not a single evolution item.
Users in this thread have also been reporting how many spins it took them to find their first evolutionary item (or not):
300 for me, none yet.
100~ got 1 King's Rock
200+ nothing. I did get a king stone from my 7 day streak reward.
Within my first 4 in the way to the station this morning, i received a Sun stone. Nothing since.
A fourth thread has been aggregating data to try to figure out the drop rates for the new items as well.
13 nanab berries and 6 pinap berries from probably 200 pokestop spins. No evolution items.
Got an upgrade after maybe 50 odd spins?
None after +100 spins
I received a dragon scale this morning on my 7th day streak pokestop. In the U.K. near Reading.
My son got Up-grade on first spin of the day today (not 7-day). Maryland, USA.
Spun roughly 500 Pokestops since Gen2 launched. Zero evolution items thus far but the drop rate on Pinap/Nanab is fairly good.
There are other threads like these, like this one or this one.
In addition to all of these reports, from the comments above:
Additionally, I have received 0 evolutionary items from about 10 spins since they became available.
Best Answer
Based on the data produced by this survey for people to submit their personal experiences with candy received to Pokémon fed berries ratio, the rate at which a trainer will receive candies from feeding gym defenders berries is roughly 0.65%.