Validity of self-reported height and weight among adolescents: the importance of reporting capability.

Background: This study proposes a new approach for investigating bias in self-reported data on height and weight among adolescents by studying the relevance of participants' self-reported response capability. The objectives were 1) to estimate the prevalence of students with high and low self-reported response capability for weight and height in a self-administrated questionnaire survey among 11-15 year old Danish adolescents, 2) to estimate the proportion of missing values on self-reported height and weight in relation to capability for reporting height and weight, and 3) to investigate the extent to which adolescents' response capability is of importance for the accuracy and precision of self-reported height and weight. Also, the study investigated the impact of students' response capability on estimating prevalence rates of overweight.

Methods: Data was collected by a school-based cross-sectional questionnaire survey among students aged 11-15 years in 13 schools in Aarhus, Denmark, response rate =89%, n = 2100. Response capability was based on students' reports of perceived ability to report weight/height and weighing/height measuring history. Direct measures of height and weight were collected by school health nurses.

Results: One third of the students had low response capability for weight and height, respectively, and every second student had low response capability for BMI. The proportion of missing values on self-reported weight and height was significantly higher among students who were not weighed and height measured recently and among students who reported low recall ability. Among both boys and girls the precision of self-reported height and weight tended to be lower than among students with low response capability. Low response capability was related to BMI (z-score) and overweight prevalence among girls. These findings were due to a larger systematic underestimation of weight among girls who were not weighed recently (-1.02 kg, p < 0.0001) and among girls with low recall ability for weight (-0.99 kg, p = 0.0024).

Conclusion: This study indicates that response capability may be relevant for the accuracy of girls' self-reported measurements of weight and height. Consequently, by integrating items on response capability in survey instruments, participants with low capability can be identified. Similar analyses based on other and less selected populations are recommended.