08/17/2011 12:22 EDT | Updated 10/17/2011 05:12 EDT

Kids' obesity rates skewed by parents' estimates

Canadian parents underestimate the heights and weights of their children, which skews estimates of obesity rates, a new report suggests.

Statistics Canada released two reports on Wednesday comparing the bias in Canadians' perceptions about their weights and what they actually weigh.

The report on children aged six to 11 suggested that on average, parents underestimate the height of their children by 3.3 centimetres and weights by 1.1 kilograms or 2.4 pounds.

The body mass index — or BMI — that is often used to determine obesity is a measure of weight in relation to height.

"It's interesting because when you look the bias in body mass index it's not that great. But it's just that the two biases are upsetting each other because they underreport both the height and the weight. So we end up with a bit of a mess, with the wrong children being classified as obese," said Margot Shields, a senior analyst with Statistics Canada in Ottawa.

"It tells us if we want to get an accurate picture we need to have measured data."

The tendency of parents of overweight or obese children based on measurements to underestimate their child's weight was fairly consistent across studies, the report's authors said.