Body Mass Index (BMI) is an index of weight-for-height that is used to classify underweight, overweight and obesity in adults. It is defined as weight in kilograms divided by the square of height in metres (kg/m2).
The validity of self-reported BMI data is a long-standing question posed by researchers from around the world. Some studies indicate that self-report data is not a valid measure due to social desirability bias or lack of knowledge or recall. Specifically, weight tends to be under-estimated while height is over-estimated, which can lead to a misclassification of BMI status. The Health Promotion Agency (HPA) collected self-reported height and weight data in its 2012 Health and Lifestyles Survey (HLS) while the Ministry of Health collected measured height and weight data in its 20011-12 New Zealand Health Survey. As both surveys are nationally representative and methodologically rigorous, their findings are comparable for analytic purposes.
Appendix A - BMI Comparison HLS Health Survey [MS Excel, 699 KB]