A study from aQuantive's Atlas Institute looks into the effect of overlap of ads, either across publishers or across multiple ad placements. The study looked at 16 large advertisers in the first quarter of 2007, and found that a minority of users are reached across multiple sites, but that group is responsible for the majority of conversions.
A user reached across multiple publishers is twice as likely to convert as one reached by a single publisher, an 86 percent of converters saw a placement other than the last ad seen.
What this means for search marketers is that they should not rely on counting the last ad seen to measure results. This idea of "assists" has been talked about for some time now, and Yahoo has built it into its analytics product.
It also means that marketers should measure overlap in every campaign, and maximize its effect for branding or minimize its impact on frequency, which could negatively impact direct response campaigns.
ClickZ News has more details.