At Pocket Gamer Connects San Francisco 2015, Jeff Gurian, Director of Marketing at Kongregate, gave a talk on "dark metrics."
He began by talking about the importance of icons, and said that "testing should be comprehensive and it should always be ongoing" to make sure you're reaching the most users.
Missing the larger point
Gurian also warned that if you're simply increasing your CPI budget without seeing any kind of changes in user acquisition, "you might be missing the larger point that no one's clicking your ad."
He also gave some advice on using ad mediation, suggesting that developers shouldn't just stick to automatic settings and instead "play around with your priorities" to see what works best.
You can see all our videos from Pocket Gamer Connects San Francisco 2015 in this YouTube Playlist.