Take a look at conversions for Google search vs. Google Partners. It seems that quality of partners' traffic has been going down over time -- at least in my stats.
We just paused most of our campaigns on the partner network or dramatically reduced bids.
This is interesting - about Amazon's departure in late 2014 and onboarding Wal-Mart
http://searchengineland.com/amazons-departure-will-impact-googles-search-network-203172
related: Placement targeting is too cheap. I've seen sites having their book rate undercut by 80% when the same inventory goes through adwords. You can control the quality that way too.