In the National Health Survey/Leefstijlmonitor overweight is based on the so-called Body Mass Index (BMI). The BMI compares a person’s weight to their height. In the survey respondents are invited to report their body height and weight. In the case of children under the age of 12, the parent/carer reports body weight and height. The questions is formulated as follows:
- What is your/your child’s height? Height (in centimetres) without shoes.
- What is your/your child’s weight? Weight (in kilograms) without clothes.
The BMI is defined as the body weight (in kg) divided by the square of the body height (in m). A BMI of 25 or more is an indication of overweight. A BMI of 30 or more indicates serious overweight (obesity). For under-18s different criteria apply which can be found in the article Establishing a standard definition for child overweight and obesity worldwide. International survey.
Body height and weight are self-reported and not measured by a physician or medical researcher. This procedure may lead to underreporting, as is shown in the article Vertraging in lengtegroei en gewichtstoename (Dutch only).
If the respondent’s height and/or weight is unknown or if the relation between weight and height is unlikely, the data are not included in the survey. In the period 1981-2013 persons aged 20 years or older with a BMI score of less than 14 kg/m2 or more than 45 kg/m2 and persons under the age of 20 with a BMI score of less than 10 kg/m2 or more than 45 kg/m2were not covered by the survey. From 2014 onwards, the BMI score for people of all ages has been raised to 50 kg/m2. The BMI lower limit has not changed.
The analysis for 2015 includes 2,253 under-25s and 6,543 persons aged 25 years and older. The data presented in the survey are sampled and therefore are subject to a margin of error.
A different method for measuring overweight is based on waist circumference. In fact, BMI and waist circumference (percentage of body fat) give the same indication.