NBC Universal has sent a letter to Nielsen expressing concern over its Total Content Ratings product and asking the media measurement giant to delay its introduction.

In a letter obtained by Ad Age, Linda Yaccarino, chairman-advertising sales and client partnerships at NBCU, said that the Total Content Ratings product in its current form "lacks the consistency and transparency the marketplace demands and expects from Nielsen" and is not ready to be released.

As TV networks continue to see substantial ratings erosion among traditional viewers, they have been waiting eagerly for a measurement system that counts all viewing no matter where it takes place, including streaming platforms and mobile devices, in the hopes that they will discover some of the audiences that have gone missing.

Nielsen has said that its intended solution, Total Content Ratings, is on schedule to begin providing syndicated data to clients on March 1. That means Nielsen customers will be able to use the data for public release. Today the data is being released only to publishers and agencies for internal use.

But Ms. Yaccarino wrote in the letter that there are several concerns with the system, including limited participation and implementation across the industry, with some leading pay-TV operators and digital video distributors not incorporated; the use of "disparate digital data collection methods using a mix of panel, census and custom measurement (YouTube) that is not yet vetted"; and "opaque" Facebook data for duplication of audiences that is not tested.

Ms. Yaccarino added that the product uses consumer panels to extrapolate video on demand audiences, which she said requires direct digital measurement instead; provides partial and incomplete representation of over-the-top viewership; lacks a standardized reporting hierarchy across all measured entities; fails to measure out-of-home viewing; and has no quality control measure for digital fraud, viewability or completion rates.

"The whole industry eagerly awaits total audience data, and we recognize no solution will be perfect," Ms. Yaccarino wrote in the letter. "However, a reasonable degree of accuracy and transparency must be minimum requirements for the launch of TCR. TCR is far from meeting these requirements, and it is premature syndicated release is irresponsible."

A Nielsen spokesman said the company stands behind its total audience measurement.

"Since January 2016, Nielsen has been working with publishers through various implementations of the components of Total Content Ratings and Digital Content Ratings. Currently, dozens of TV and digital media brands are enabled for TV, VOD or digital measurement inside of Nielsen's Total Audience measurement framework," the spokesman said in an email. "Nielsen does not stipulate which measurements clients should enable, nor the order in which they should enable them. Total Audience Measurement is designed to provide media owners with utmost flexibility to enable the components based on their business priorities."

Since the beginning of the year, Nielsen has met regularly with members of its senior research council, which is comprised of 25 TV and agency clients, to discuss the implementation and rollout of total audience measurement, he said.

"We continue to enhance and refine our product with ongoing updates as we work with clients during this period of evaluation," the spokesman added. "Nielsen is working across business functions with our clients during this period of evaluation. We do not underestimate the importance of the rigor of this work, as clients use our metrics to make decisions on content, programming, distribution, and as a currency for ad planning and settlement."

NBC Universal does not appear to be alone in its worries.

"We share many of the same concerns NBC expressed in the letter," said Joe Marchese, president-advanced advertising products, Fox Networks Group. He also said he was concerned about how Nielsen will define digital viewing and take into account things like share of screen, whether the sound is on and how long an ad runs.

Viacom has also shared reservations similar to NBC's, according to a person familiar with the situation.

Implementing the technology to measure all eyeballs has been difficult and tedious, according to the person, and needs to be done for each individual brand in a portfolio of networks. Because not every network has done it, what Nielsen will release "is going to be apples to oranges," the person said.

"I have never heard this amount of concern echoed across the industry about anything, ever," the person said.

The timing of the release of Total Content Ratings in March is also worrisome to network executives, because it falls right before the all-important upfront selling season. The fear is agencies will use TCR to make decisions about where to spend marketers' TV dollars -- and how much to spend.

At the same time, ComScore, which merged with Rentrak in February, has been working to develop its own cross-screen measurement, but that too has its issues, executives said.

Brian Wieser, anaylst at Pivotal Research Group, said in a research note Thursday that he had heard "mixed messages (to put it mildly) from both advertisers and agencies about the current state of the product -- both the process by which the data is gathered and whether the data is reflective of the actual consumption of content."

Still, Mr. Wieser said he believes that in whatever form it takes, Nielsen's metrics will still be regarded as the industry-standard measure of video consumption, and that any flaws that may exist in the product will eventually be fixed.

"Ultimately, the perspectives that matter most here are not those of the networks, but of agencies and marketers, and so far as we are aware, few of those stakeholders have had significant exposure to the TCR product," he wrote. "Further, we were skeptical that the new measurement platform would meaningfully impact the amount of new money that network owners would see…Changes in the measurement of supply of inventory do not meaningfully alter demand. However, the concept of TCR was probably the best initiative we are aware of that would have helped national TV owners explain to stakeholders (marketers and investors in particular) that consumption of their content is not declining, as conventional C3-based ratings would suggest."