Bots are only the tip of the iceberg with the NHT [Non-Human Traffic] problem, and the deeper you dig the more interesting it gets. comScore has observed secondary sets of activity, generated from users’ computers while consuming media, that exists in a parallel thread on the user’s computer that they never see. These processes have an explicit intent of driving incremental usage to publisher sites… Recent studies have estimated bot traffic to be anywhere from 4% to 31% of total web traffic in the U.S., which translates to anywhere between $650 million and $4.7 billion of wasted marketing spend.
No link to studies but, even if those are ballpark estimates, that would be on par with email spam rates in the early part of last decade. Last fall, AdAge detailed measurement techniques used by comScore, Nielsen, etc. and Compete openly details how their data practices differ from competitors. Trends like NHT will only increase uncertainty.