Advertising overlap on multiple sites has a significant impact on consumer conversions, according to "How Overlap Impacts Reach, Frequency and Conversions," a study by aQuantive's Atlas Institute, writes MarketingCharts.

The study finds that consumers are more likely to convert after viewing ads on multiple websites; therefore, it would be more accurate for marketers to attribute conversions to a full set of impressions and/or clicks rather than the current industry standard of attributing all online conversions to the last impression (or last "click"), according to Atlas.

Some key findings from the study:

• Consumers reached across multiple publishers were twice as likely to convert as those reached solely through a single publisher.
  
• 90 percent of the consumers that converted were reached by placements other than the last ad seen.
  
• Two out of three consumers who eventually took a responsive action were reached by ads across multiple portal sites before converting.

A previous Atlas Institute study, "The Combined Impact of Search and Display Advertising," found that sponsored search and display advertising together provide a 22 percent higher conversion rate over search alone.