From: http://www.nielsen.com/us/en/insights/news/2014/the-value-of-efficient-reach-maximizing-campaign-audiences.html
The main factor in terms of a campaign’s reach efficiency is sites’ ability to serve and optimize ads effectively.
Reach have always been a critical goal for advertisers. Reach is how many unique persons are exposed to an ad.
According to an analysis of Nielsen Online Campaign Ratings data in 2014, 59 percent of ad impressions served across all consumer segments reach their intended audience—down from 69 percent in 2013. Which correlates with a shift toward narrower, more focused campaign audiences from broader demographic segments, according to Nielsen. But this on-target delivery decrease is only part of the story. Let´s look at Nielsens evaluation of reach efficiency:
Reach efficiency refers to how efficiently a campaign and its individual media partners reach unique audiences within a specific demographic group given a set level of gross rating points (GRPs)
Despite having similar parameters and goals, campaigns can perform differently based on the sites they’re served on. The main factor in terms of a campaign’s reach efficiency is these sites’ ability to serve and optimize ads effectively. Keeping this idea in mind is critical for advertisers to understand whether Reach goals are met and which sites are contributing the most to this goal.