At Pocket Gamer Connects San Francisco 2015, Sebastian Kriese, Head of Partnerships at Wooga, gave a talk about moving beyond simply looking at your game's LifeTime Value (LTV).
He said that for Wooga, ""75-80% of traffic and installs we're receiving are not trackable", which makes it hard to really understand your CPI versus your LTV.
The majority of these installs are instead "organic", but it's entirely possible that while they were not direct installs from ads, they could have been influenced by an ad campaign.
For example, a player may look at an ad and not click through, but decide to download the game later from the app store, which makes it impossible to track the install.
You can see all our videos from Pocket Gamer Connects San Francisco 2015 in this YouTube Playlist.