How many advertisements can U.S. TV networks jam into their commercial breaks? According to the tabulations of two Wall Street analysts, too many.

Beset by declines in audience, a majority of U.S. cable networks stuffed more commercials onto their air in the fourth quarter, with Viacom boosting its ad load by 13% across its cable networks; A+E Networks increasing the number of commercials it runs by 10%; and Discovery Communications adding 9% more TV spots, according to research released Wednesday by independent analyst Michael Nathanson. On the broadcast side, Fox raised the number of spots it aired by 15% in the quarter, Nathanson said, while ABC and CBS reduced theirs by 2% and NBC cut its by 6%.

Executives for Discovery, A+E, Viacom and Fox were not immediately available for comment.

TV networks have been caught jamming more ads into their schedule in the past, and it’s not a move borne from confidence. TV networks guarantee their sponsors a certain number of viewer impressions, and when ratings fall, they are often contractually bound to give their clients additional commercial time, often known as a “make good,” as recompense for missed targets. One alternative? Running commercials again and again in order to reach additional viewers and thus avoid ceding more inventory.

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The reports – Bernstein Research analyst Todd Juenger has also released a study of the phenomenon – come just before the media industry is to release information about fourth-quarter performance to investors. And they come as both Wall Street and Madison Avenue are growing more concerned about TV’s ability to capture ad dollars in the face of new video alternatives such as mobile devices and streaming-video hubs.

Ad spending on TV in the fourth quarter fell by 2%, according to Standard Media Index, a company that tracks U.S. ad spending by examining the bookings of media-buying firms that it says represents 80% of the market. Among the factors was a 9% dip in overall ad spending by automotive advertisers, the largest spender on all types of advertising during the period.

The question about ad stuffing surfaces as some media companies are trying to work out new agreements with advertisers that may rely on measures not based on the industry’s current foundation, age and gender viewer calculations from Nielsen. In a recent call with investors, Viacom CEO Philippe Dauman recently spoke of an effort to create advertising models that have new metrics at their core – the better to try to count the younger viewers who make up so much of Viacom’s base and are more prone to use new video-streaming technology not currently measured fully. NBCUniversal’s CNBC recently announced it would, starting in the fourth quarter of 2015, no longer use Nielsen to measure the audiences for its daytime business-news programming.

Adding more commercials might disturb the advertisers who pay for them, said Juenger (to say nothing of viewers). “One wonders why the agencies let the networks get away with the ad stuffing,” he wrote in his research report. “Surely the ‘stuffed’ ads are piling on frequency, not increased reach, among the same group of brain-dead viewers who are happy to watch repeat content with even more ads than the already onerous U.S. average. That doesn’t seem like an attractive audience target for many brands, except maybe certain infomercial products.”

People familiar with some of the TV networks in question took issue with the analysts’ data. Fox has not increased its ad load, according to a person familiar with the network. Another person familiar with the matter suggested the ad-load calculations may be inflated.

One factor in the tabulations may have been the desire on part of some advertisers for 15-second ads instead of 30-second ones, which would lead to a higher number of commercials that could be tabulated. It is also unclear whether the counts include only national commercials or such things as ad time given to local stations, network promos and public service announcements, these people said.

Even so, a calculation by Nathanson of air time devoted to commercials (as opposed to the commercials themselves) in the fourth quarter shows increases at some cable outlets.  He noted the hikes in that metric were still high for Viacom, up mid-to-high single-digit percentages.  At Discovery Communications, for example,  commercial minutes rose in the low-single-digit percentage range in the quarter, Nathanson said. At Fox Broadcasting, commercial minutes wre found to be flat with or down from the number run in the year-earlier quarter.

Viewers have endured ad-jamming in other instances. The tactic typically shows up during lower-rated content, like Friday-night reruns or movies screened on weekend afternoons. In 2010, Viacom’s Spike was caught running ad breaks lasting anywhere from six to 10 minutes during poorly watched Friday-night repeats of HBO comedy “Entourage.” In some cases, the commercial breaks were longer than program segments from the shows.

Correction: A day after publishing his data, Nathanson said his figures on air time devoted to commercials were incorrect. Viacom ad minutes, originally thought to be up 8%, were instead up in what Nathanson said were “mid-to-high single digit percentages. At Discovery Communications, ad minutes said to be up 5% were actually up in a low-single-digit percentage range. Fox Broadcasting’s ad minutes, originally said to be up 12%, were actually flat to slightly down. Nathanson has retracted that data and this article has been updated to reflect his new findings.