Understanding the Impact of Nielsen’s Methodology Switch

Nielsen’s switch from a panel-based methodology to include “big data” sets is finally becoming a reality after years in the making. Nielsen has been reporting TV viewership data using both the “Panel Only” and “Big Data + Panel” methodologies for over a year now, but they will be sunsetting the “Panel Only” data at the end of the year. This methodology change has implications for all television advertisers, so Rain the Growth Agency did a deep dive into how impressions are varying with the new methodology, and how that might impact the video landscape in the year ahead.

Why is This Happening?

Nielsen held a monopoly for television viewership measurement for decades. Nielsen has relied on a panel of people meter devices (and before that, hand written diaries) in 42,000 households to represent about 125 million TV households in the U.S. More recently, competition arose from companies like VideoAmp, Innovid, and ComScore. These competitors primarily use “big data” available from cable MVPD set-top boxes (STBs) and Connected TV (CTV) automatic content recognition (ACR) software. What makes it “big data” is that these data sets include millions of devices and households, not thousands.

In response, Nielsen developed a product that combines both panel data and big data to help alleviate the concerns around under-representation of both methodologies. It takes the existing panel and adds STB/ACR data from over 75 million devices (Smart TVs or STBs) in 45 million households. Nielsen has been testing this methodology for several years and announced that they would be sunsetting the panel-only data by the end of the year.

What Does the Data Show?

Rain the Growth Agency analyzed years of both data sets and compared the differences in impression counts between the soon to be obsolete “Panel Only” data and the new “Big Data + Panel” data. Here’s what we found:

• Big Data + Panel impressions were consistently higher than the reported impressions in Panel Only. For 2025 year-to-date, the Big Data + Panel shows over 1.5 million more persons 18+ watching linear TV than the panel only was reporting.
• The gaps between the methodologies have narrowed since the Big Data + Panel methodology launched. In some demographics, like persons 25-54, there’s no difference between the methodologies’ reported viewership in 2025, while others, like persons 50+, still show Big Data + Panel impressions to be about 5% higher than Panel Only.
• While the aggregate numbers are relatively close, there are big differences when you look at individual networks. The big broadcast networks ABC, CBS, NBC, FOX all saw their viewership increase with the new methodology. ABC and NBC each gained over a million viewers in the switch to Big Data + Panel reporting.
• The biggest gains came to relatively small networks, like CNN en Español who saw their audience grow 50% versus their panel only audience.
• Many of the largest gains were realized by Spanish-language networks, indicating that the previous panel only approach was undercounting Hispanic viewers.

What are the Implications for Advertisers?

It is important to remember that a change in measurement is not the same thing as a change in viewership. The same is true of performance outcomes: website visits, phone calls, downloads, purchases, etc. The same number of viewers who saw an ad and took action because of that ad hasn’t changed.

Impression data is used to calculate some important media metrics, and those will be impacted by the switch to Big Data + Panel measurement. Response rates—the percentage of viewers exposed that completed a desired action—will change in reverse proportion to the change in impressions. If a network or show’s impressions increase with the methodology change, then the response rate will decline (and vice versa). The same is true with CPMs, since higher impressions at the same cost will lead to lower CPMs (and vice versa). Finally, for advertisers who rely on spike attribution methodologies, like Rain the Growth Agency’s SignalLink product, there could be some changes in attribution when multiple ad instances occur near a spike in outcomes. The unit with the higher impression count receives the attribution for that spike, and it is conceivable that Big Data + Panel impressions could indicate a different winner in the tie-breaker scenarios.

The implications for the marketplace remain to be seen, as adoption of Big Data + Panel impressions are just beginning.

1. One impact is that many networks will have an easier time making their guarantees to upfront advertisers with fewer audience deficiency units (ADUs) required to make up for underdelivery. Fewer ADUs mean more inventory available in the scatter market, creating more opportunities at lower costs.
2. While the Big Data + Panel impressions are higher in aggregate, some networks and dayparts are seeing lower impressions with the new
   methodology. So, while it may create favorable scatter opportunities for most networks, there will be some that will see greater underdelivery and require more ADUs to make upfront advertisers whole.
3. Finally, the methodology change requires that future upfront guarantees be based on Big Data + Panel impressions, when they were previously built on panel only impressions. This will muddy year-over-year rate of change analysis, with some CPM increases being offset by increased/decreased impressions due to the new methodology.

We believe that Nielsen’s shift to the Big Data + Panel methodology is the right move for the marketplace, even if it is a little overdue. Based on the comparative data that we analyzed, the new data stream is better suited to measure more diverse audiences and increasing viewership options those audiences have. As advertisers and agencies begin using this new currency, below is a list of what advertisers should think about to prepare for Nielsen’s Big Data + Panel updates:

1. Understand the Timeline

• Sept 1, 2025: Big Data + Panel became the official currency for national TV transactions.
• Jan 1, 2026: Panel-only Automatic Content Measurement (ACM) will be removed from transactional systems. Historical data will still use panel-only for prior years, so mixed methodologies will exist.

2. Audit Current Campaigns

• Review performance projections and pacing under the old panel-only system and understand how the new methodology may change things.
• Identify campaigns that will span Q4 2025 into Q1 2026, as these will require methodology adjustments.
• Understand impact on ROI, if any: How will Big Data + Panel affect reach, frequency, and cost-efficiency metrics?
• Historical Comparisons: What normalization methods will be used to compare panel-only vs. hybrid data?

3. Prepare for Mixed Methodologies

• Expect shifts in ratings and rankings—sports and streaming-heavy programs may show significant gains.
• Historical trend reports may still rely on panel-only data for prior years, so confirm which data sets you are comparing.

This article is featured in Media Impact Report No. 70. View the full report here.