Apple's dominance on tablets and smartphones presents a threat to accurately measure and optimize the performance of paid-search marketing campaigns. Search firm Marin Software published a white paper Tuesday based on findings and unanswered questions surrounding Apple's iOS platform. The report identifies Safari, the primary browser for iOS devices, as a major challenge because it blocks third-party cookies by default, making it difficult for ad servers, tracking systems, and ad management tools to link visitors to ads that brought them to the Web site. Piper Jaffray Analyst Gene Munster estimates Apple will sell 1 million iPad 2s faster than the 28 days it took to sell 1 million of the first-generation iPad. And more people continue to adopt Apple products. During the first weekend of sales, the analyst firm found that 70% of iPad 2 buyers were new to the iPad, compared with 23% of iPhone 4 buyers who were new to the iPhone at launch. While Mac users also rely on other browsers, Safari remains the dominant search browser used on the iPhone and the iPad, which results in higher rates of undercounted conversions on Apple devices. All browsers can present challenges for advertisers, but Apple's focus on consumer privacy limits the viability of third-party cookie-based tracking systems. Marin's research also suggests that the conversion tracking issue is a much bigger problem than previously thought. On average, advertisers using third-party cookie-based tracking systems are undercounting conversions by 38%, severely limiting visibility into campaign performance. The white paper, however, does provide somewhat of a workaround. Blocking third-party cookies can make iOS conversion rates appear lower than conversion rates on Windows, but the study found that the actual conversion rates for iOS, minus for the third-party cookie based undercounting, were on average 23% higher than on Windows. To demonstrate the point, Marin "indexed paid-search conversion rates to a baseline value of 100 for the Windows platform. For the same dataset, the company analyzed iOS conversion rates when third-party cookies were blocked and when they were accepted." Marin estimates that its clients contribute about 5% of converting traffic originating from an iOS device. While that's not significant today, the search marketing firm points to growing demand for the iPad and its "high-quality browsing experience." Nonetheless, more consumers continue to adopt Apple products, and this poses challenges for advertisers when users search the Web using the Safari browser.