New York—Media spending on multiscreen ad campaigns will grow from 20% of budgets this year to half of media budgets within three years, according to a study by the Association of National Advertisers and Nielsen Co. The study was based on an online survey of 274 marketers, media buyers and ad agency executives conducted in July and August. It found that 88% of respondents said ad campaigns running across two or more screens—including TVs, computers, tablets and mobile phones—will be very important in three years. Measuring multiscreen campaigns is a challenge for marketers, according to the study. The majority of respondents (71%) said they now use a variety of metrics specific to individual screens to measure the effectiveness of ad campaigns. However, 73% said they would prefer to use just one set of metrics to measure the effectiveness of ad campaigns running across multiple screens.