Projected ad growth for 2013 has been cut slightly by ZenithOptimedia. The firm believes a 3.9% increase is mostly likely for this year, a number which is down from the 4.1% rate analysts had been expecting at the end of last year. Countries in the Rising Markets category will contribute the most to ad market growth through 2015, however, the U.S., which is a Mature Market, will remain the top market for ad spending.