AI is changing where people discover products and how they move toward a purchase. The problem is that advertisers’ measurement systems aren’t changing nearly as fast.
That’s a major media investment headache, according to IAB’s “2026 Outlook Study: September Update.” The survey of 211 U.S. brand and agency ad investment decision-makers found 44% cite adapting to changing consumer behavior, including AI-driven search, as a top media investment challenge.
There’s more money riding on those decisions, too. IAB raised its forecast for U.S. ad spending growth this year from 9.5% in January to 12.3% in September. So advertisers are spending more while AI is making it harder to understand discovery and what influences purchases.
Marketers are adapting faster than they can measure
Advertisers’ practices are already changing due to AI-driven discovery. Seventy-six percent of marketers said optimizing content for AI-generated answers is what they’ll be focusing on the most, followed by AI LLM models at 72%. Meanwhile, increased focus on using generative AI in media campaigns fell from 78% in January to 69% in September.
The trouble is figuring out whether any of it is working. Forty-five percent of buyers say comparing AI-driven and traditional customer journeys is one of their biggest measurement challenges related to conversational AI and agents. Another 35% struggle to get consistent data on brand visibility and citations in AI tools, while 30% cite missing or unreliable AI referral data.
Advertisers aren’t waiting for someone to solve the measurement problem for them. Eighty-six percent are changing how they measure media performance because of AI and agents, or expect to do so in the next 12 months.
For now, that means cobbling together different signals. Forty-eight percent measure brand visibility and citations in AI tools, 44% use branded search and direct traffic as proxies, and 40% use third-party AI discovery analysis tools. Another 30% are increasing their use of incrementality tests, and the same percentage are using modeled measurement.
And the old metrics aren’t going away. Only 26% of buyers are putting less weight on website traffic. Instead of replacing the existing measurement system, AI is giving marketers another set of metrics and tools to layer on top of it.
AI is also changing where the money goes
Retail media is a good example of how quickly spending can move, even when measurement is still catching up. IAB expects commerce media spending to grow 13.6% this year, up from its January forecast of 12.1%. The report says AI is contributing to that growth by shortening the distance between discovering a product and buying it.
There’s another wrinkle: Marketers also have to figure out who — or what — is interacting with their content. Twenty-seven percent of buyers list bots and agents outnumbering humans in web traffic as a media investment concern. And when it comes to measurement, 28% struggle to distinguish humans from legitimate agents, while 33% struggle to tell legitimate agents from bots and fraud.
That leaves advertisers trying to follow consumers into AI-influenced buying journeys while sorting out which interactions come from people, which come from agents working for people, and which are just bots.
The complete report can be downloaded here. (Registration required)
