eCPM is a metric I look at, but fill rate is also important. When I use second tier mobile ad services, the eCPM is often competitive, but my fill rate can dro

eCPM is a metric I look at, but fill rate is also important. When I use second tier mobile ad services, the eCPM is often competitive, but my fill rate can drop from 99% on a big service to <10%. So over 90% of my ad requests go unfilled...if I was solely using a low (<40%) fill rate service on an app, even if eCPM was double I'd be losing money. Even if you can get good, independent mediation with per-service, per-country settings and good backfill - which itself can be difficult - it can take a couple of seconds for that second ad backfill to come in, which usually means a dropoff in clicks. Whenever I see second tier ad services boasting good eCPM, I always wonder what their fill rate is, especially if you're app isn't something you'd expect lots of ads for, like information on cars for people in the USA.