Shortly after the new gym system was released, reports flooded in that feeding berries to gym defenders would sometimes reward a candy of that species’ family (alongside the standard reward of 20 Stardust). 

Interestingly, this bonus candy drop is rather uncommon, so the precise probability has remained unknown. The recent efforts of the Silph Research Group, however, have shed some light on this topic.

Findings

Over the past few months, 240 researchers fed over 100,000 berries in a controlled study and received over a thousand candies for their valiant efforts! Using this data, we are able to say with 95% confidence that the expected chance interval of receiving a candy from feeding a defending Pokémon a berry is:

(1.20 ± 0.06)%

Or roughly 1 in 85 berries.  Here’s a look at the raw numbers:

We also examined the potential influence of the following external circumstances:

• Gym Medal (Vanilla, Bronze, Silver, or Gold)
• Berry Type (Razz, Golden Razz, Nanab, or Pinap)
• Species
• Remote Feeding
• The defender being your own Pokémon
• The CP increasing after feeding

We found that NONE of these variables have any observable effect on the candy drop rate!

Methodology

Each berry fed in this study was also recorded alongside several possible influencing factors (mentioned above): the trainer’s gym medal rank, the berry type, the species being fed, whether it was fed remotely, whether the researchers were feeding their own Pokémon or another trainer’s, and whether motivation was restored by the feeding or not. We checked whether the rate of awarded candy was independent of each of the potential influencing factors with a Pearson’s Chi-squared Test, with an alpha value set at 0.05. If the p-value of the test is greater than 0.05, then there is not sufficient evidence to conclude that the factor has any influence on candy drop rates.
This is what we found:

Pearson’s chi-squared test p-value = 0.365

Pearson’s chi-squared test p-value = 0.137

Pearson’s chi-squared test p-value = 0.088

Pearson’s chi-squared test p-value = 0.238

Pearson’s chi-squared test p-value = 0.884

Pearson’s chi-squared test p-value = 0.808

* Species list filtered to only include those with 3000+ data points

Parting Words

With these findings in hand, our travelers now know how often bonus candy should appear, and that there is no need to worry about strategizing to increase your bonus candy drop rate! Just remember that each berry has an independent chance of receiving a candy drop. Receiving 2 or 3 candies in a short time period as well as experiencing several hundred candy-less feedings are very possible. It’s easy to imagine a “lucky” species or berry, but …it’s all a random pull from a hat!

Happy berrying, travelers.

Note

Some have hypothesized that the bonus candy drop rate may be altered during or after events.

Fortunately, the data collected spanned the Halloween event, including the pre- and post-event periods.  After grouping and examining the relevant periods, it can be concluded that the drop rate has not changed over the pre- and post-Halloween periods.

Pearson’s chi-squared test p-value =0.363 

* The number of berries does not add up to the total of 113,502 as time periods around the event changes (Oct 21, 7pm UTC and Nov 2, 8pm UTC) have been excluded to ensure data integrity