An analyst who thought he had discovered many big U.S. TV networks stuffing more commercials on to their schedules said one day after issuing a report on the subject that some of his findings were based on erroneous data.

Michael Nathanson, who studies the media industry for the independent MoffettNathanson firm, used TiVo and other data to determine that the Fox broadcast network had increased its commercial minutes in the fourth quarter by 12% over the year-earlier period, that Viacom-owned cable outlets had increased their overall commercial minutes by 8%, that A+E Networks had increased them by 6% and that Discovery Communications outlets had increased them by 5%.

Now, said Nathanson, in a note, he believes the data is flawed.

“After following up with a handful of our companies, we believe that Fox Broadcasting was the most misrepresented with TiVo data implying a +12% increase in the past quarter versus flat-to-slightly down.  A massive error,” Nathanson said. “Within cable networks, TiVo’s estimate of +8% for Viacom did not include all of their cable networks.  After adjusting for all of Viacom’s cable networks under management, the number of commercial minutes increased, but at a lower rate, up mid-to-high single digits.  For Discovery, commercial minutes is closer to up low-single digits, not the +5% tracked by TiVo.  We note that these revised figures have been corroborated by Nielsen’s Monitor Plus service.”

Nathanson did not say some of his other findings, based on a look at the number of commercial units, were in error. He did acknowledge that increases in commercial units at various networks could be the result of a new desire for 15-second spots rather than commercials that are normally 30 seconds in length.

Variety published a story Wednesday on the issue of “commercial stuffing” that asked whether TV networks were running ads multiple times in order to meet certain audience-impression guarantees.  A second analyst who also looked into the trend in a research note yesterday, Todd Juenger, said via email Thursday night that “we completely stand by our work.”