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When It Comes To Exercise, Don’t Trust. Verify.

Researchers studying obesity have a problem: People seem to lie about how much they work out.

Late last month, the national Health and Social Care Information Centre (HSCIC), the main health-information agency in England, reported that adult obesity rose by about 70 percent from 1993 to 2012, even as people, on average, said they were meeting workout targets and eating fewer calories than the recommended totals.

What’s going on here? As The Guardian highlighted, obesity data come from direct measurement (pretty good), and the diet and exercise figures come from survey responses (not so good).

Obesity measurements aren’t perfect — body mass index cutoffs can ensnare the super-fit as obese — but at least researchers get them directly. In polls, people are likely to tell interviewers what they want to hear and what they want to think of themselves.

For example, here’s one alarming finding from a 2008 report by the HSCIC: Fewer than one in 10 English adults who said they were meeting exercise guidelines really were. (See Page 70 of this PDF.) The directly measured exercise data came from accelerometers people wore; their self-reported figures were from the prior four weeks. So even when people knew they were being measured against their own claims, almost no one lived up to them — one reason to think even that paltry one-in-10 figure might be too high.

Here’s another reason to think activity levels could be lower than measured: Fewer than half of those given accelerometers wore them more than half the days they were asked to. Who’s most likely to stop recording data for researchers? Here’s one untested hypothesis: Those who know their data will show they’re accelerating much less than they just told those same researchers.

Exaggerating workout time isn’t uniquely English. Researchers in the U.S. and Norway have found similar results.

Carl Bialik was FiveThirtyEight’s lead writer for news.