Nielsen’s TV yardstick has been found wanting.

A review of recent Nielsen efforts to map out TV audience activity determined the company likely undercounted TV audiences in February of this year, a development that is expected to intensify a growing rift between the measurement giant and the TV networks who depend on it to set discussions with advertisers that help undergird billions of dollars in advertising.

The Media Rating Council, an industry organization that maintains research standards for the media industry, found a “generally consistent pattern of underreporting of viewing” in Nielsen’s reporting in February, 2021, the group revealed Monday. MRC found that total usage of TV in February by people between 18 and 49 — the audience most desired by advertisers in traditional TV entertainment — may have been understated by 2% to 6%.

The announcement is bound to open the measurement giant to new scrutiny from TV networks who believe the company’s decision to stop maintaining some of the homes in the panel of consumers it uses to set its ratings has led to significant shortfalls in TV audiences over the past year. There is “a heck of a lot of money” at stake, said Sean Cunningham, CEO of the VAB, a trade group that represents the TV networks to Madison Avenue. “There is the potential for a lot more understatement.”

At issue, TV executives believe, is a decision by Nielsen to stop sending field agents to Nielsen homes during the coronavirus pandemic. The networks fear that means Nielsen’s sample started to include homes whose residents may have moved elsewhere in the country, leaving the company tabulating results from homes where no TV watching was taking place. The executives are also concerned that a lack of maintenance could have left some homes’ results not being tracked adequately. Last week, Nielsen informed clients that it was still working to “resolve outstanding maintenance needs” in approximately 2,700 homes that serve in its panel of consumers, and would likely not complete that work until the end of May

In a letter sent to clients Monday that was reviewed by Variety, Nielsen acknowledged the discrepancies. “As a result of some of the COVID measures we implemented, we found that there was some understatement of audience estimates. The variance differed by daypart, demographic and program,” the company said.

Any miscounting may be worth millions of dollars in advertising. VAB estimates suggest that a 1% undercounting of audiences in February may represent $39 million in ad dollars, while a 6% undercounting could represent $234 million. Extrapolated over 12 months, the organization projected those figures could mean as much as $468 million to $2.8 billion.

The TV industry’s frustration on this issue is palpable. The VAB said it and several individual networks have called out discrepancies to Nielsen for months. It took “an enormous amount of unprecedented pushing” to get Nielsen to acknowledge the issues, said Cunningham. “At first blush, they have contradicted now what they have been sticking to for months.”

Nielsen presides over TV ratings at a tricky moment: More advertisers are pressing for audience measures that define TV crowds more precisely. Do they seek a new car? Are they expecting a new child? Will they visit a movie theater or download a food coupon? As the networks see their au­­diences spend more time watching ad-supported streamers such as Hulu, Peacock, Discovery Plus, Tubi, Pluto TV and Paramount Plus, they are open to testing new measures. Smaller audiences mean media companies can sell other things than the number of total views. With that in mind there is increased desire on the part of networks to use multiple measurement services and let advertisers decide which are most important.

Feuding between the networks and Nielsen has intensified in recent months. In July, Nielsen reversed a last-minute decision to delay implementation of a measure of “out of home” viewing — audiences watching TV in offices, bars, hotels and the like. The networks had already started to craft ad deals tied to it and went ballistic over the delay. Nielsen had cited the pandemic’s effects on viewership in those venues as a reason for its rethinking. In early 2019, CBS and Nielsen came to an impasse in contract talks, with the network pressing the measurement company on the cost of services and how it would measure viewers using new technologies.

In its Monday letter, Nielsen pledged to make improvements to its system, and asked for patience. “As we all continue to get back to a pre-COVID existence and the new normal, we are committed to working closely with and through the MRC to address your concerns and challenges,” the company said. A “thorough plan to address on-going panel quality,” Nielsen added, is on the agenda.