Don’t trust people when they tell you how much time they spend watching video on their smartphones and tablets. Tablet and TVThe ratings company learned that lesson the hard way after it changed the way it measures average monthly viewing from surveys to actual observations. The old numbers were off by a factor of — wait for it — 538%, TV station trade group TVB points out this morning. Using surveys, Nielsen‘s widely followed quarterly Cross-Platform Report last year put the monthly mobile video viewing average in Q4 at 5 hours and 23 minutes. But the new report, out yesterday, quietly restated that to just 1 hour — growing to 1 hour and 23 minutes a month in the last three months of 2013. “This disparity is an important reminder of the dangers inherent in trusting consumers’ claimed behavior and calls into question many recent studies that have proclaimed the demise of traditional media based on self-reported approximations of time spent with digital devices,” TVB Chief Research Officer Stacey Lynn Schulman says.

The differences by demographic are equally startling. Nielsen now says that African Americans averaged 1:20 in mobile video watching a month at the end of 2012 vs 6:15 reported last year. The 2012 Q4 monthly number for Hispanics fell to 1:30 from 5:58. And Asians last year spent 1:17, not 6:20. Nielsen doesn’t highlight the change in the table with its new numbers, although it notes toward the end of the report — in a discussion of methodology — that due to the change to its new Electronic Mobile Measurement system “data should not be trended to previous quarters’ published editions.” Online video services including Google, AOL and Yahoo are eager to win advertising dollars now spent on TV. No wonder, then, that TVB says Nielsen’s new latest measurement study shows that TV usage “has increased from 154 hours and 8 minutes per month in Q4’10 to 155 hours and 32 minutes per month in Q4’13.” It didn’t mention that the new figure is down from last year when it was 156:24.