A study, conducted in the first quarter of 2007, found that US consumers were more likely to make a purchase after viewing ads on multiple Web sites, suggesting that conversions should be attributed to a full set of impressions and/or clicks, rather than just the single one that preceded the conversion.

Two out of three consumers who eventually bought a product or took a responsive action were reached by ads across multiple portal sites before converting.

Converted* US Online Advertisements Viewed on a Single Web Site vs. Multiple Web Sites, Q1 2007 (% of total online advertisements converted)

Nine in 10 consumers who converted were reached by placements other than the last ad seen. Also, 86.1% of ads which led to a responsive action were seen on multiple placements.

Converted* US Online Advertisements Viewed on a Single Placement vs. Multiple Placements, Q1 2007 (% of total online advertisements converted)

A previous Atlas study called "The Combined Impact of Search and Display Advertising" showed that sponsored search and display advertising together provided a 22% higher conversion rate over search alone.

Of course, multiple placements are not right for every campaign, and they are not always easy to make. A June 2006 study by WebAdvantage.net found that 79% of experienced online video ad buyers were very or somewhat concerned about sizing ads for multiple placements.
Level of Concern among US Experienced Online Video Advertising Buyers Regarding Online Video Advertising Topics, June 2006 (% of respondents) 

Via eMarketer