As anyone who has ever been on a blind date knows, a lot of people fudge some of their vital statistics. Maybe he’s closer to five-foot-five than a towering six feet; maybe she’s a few years closer to forty than she suggested. These fibs are often minor, personal, and relatively inconsequential.

But when a lot of people lie about the same thing—for example, their weight—that can have serious consequences. Health care professionals and medical researchers need accurate data and statistics in order to correctly understand and treat health problems.

According to a recent government survey, the adult obesity rate in America is nearly 27 percent and increasing. Yet earlier (and more accurate) estimates put the figure closer to one-third, with no measurable increase. So why the discrepancy?

The 27 percent figure was derived from a telephone survey in which 400,000 people were asked their height and weight. Based on those statistics, a body mass index (BMI) was derived for each person, and that in turn determined whether the person was counted as obese. Since the researchers could not verify the respondent’s height and weight, the survey relied on people not only knowing their height and weight, but being honest about them—even to a total stranger over the phone.

Since people tend to both underestimate their weight and overestimate their height, this poses a problem and probably accounts for the seven percent difference. But the gap between what people say they weigh and what they actually weigh is closing; according to Dr. William Dietz of the Centers for Disease Control and Prevention, “It is possible people are paying more attention to their weight and reporting it more accurately.”

The idea that most Americans (or even American women specifically) are dieting is a myth. Most people aren’t dieting, and apparently many either aren’t paying attention to their weight (or are fibbing about it).