A new study has found that marketers aren’t sure how to measure the ROI of their mobile ad campaigns. Digital marketing news outlet ClickZ, in partnership with marketing company Search Optics, has released a survey that finds that approximately 21% of respondents do not measure the ROI of their mobile campaigns, or do not know how to do so effectively.

This uncertainty comes as market research company eMarketer released a report entitled “Need-to-Know Trends in U.S. Digital Advertising,” in which it reported that digital ad spending continues to increase in the United States, and estimated that it will make up 35.8% of total media ad spending this year, just 1% below television’s share.

Furthermore, about 46% of respondents to the study by ClickZ and Search Optics said they do not believe their mobile advertising spending correlates with their customers’ use of mobile media. Meanwhile, eMarketer estimates that 63.4% of U.S. digital ad spending this year, about $43.6 billion, is expected to move towards mobile.

Brand marketers are reallocating portions of their print budgets to serve ads on desktop as well as laptops, smartphones and tablets. Small businesses are also adjusting their budgets to offer digital ads.

In terms of implementing and measuring the return of investments on mobile campaigns, including search advertising, 56% of marketers described themselves as beginners while only 13% described themselves as advanced. Of those beginners, only 6% described their ROI as “good” or “excellent” compared with 59% for those regarded as advanced.

For search mobile campaigns, 32% of marketers said they provided good or excellent ROI while 23% said the same for social. Mobile display advertising was praised by 15%. “This isn’t to say that digital marketers haven’t taken any steps to reach a mobile audience,” Troy Smith, president of Search Optics, told Search Marketing Daily. “For example, 84% of client respondents and 79% of agency respondents report using responsive Web design.”

Smith said that marketers need to adopt advanced digital tactics such as programmatic, hyperlocal and paid search. For example, eMarketer estimates that $21.55 billion, or 67% of total U.S. display ad spending, will be transacted programmatically in 2016. Of that total, 69.1% will go toward mobile ads. Programmatic advertising includes all major methods of transacting or fulfilling digital display ads via an application programming interface.

The study by ClickZ and Search Optics surveyed 400 marketers on the state of mobile advertising in 2016.