Introduction: The aim of this study was to examine the relationship between self-reported population in order to evaluate the validity of self-reported measures for the purpose of estimating the prevalence of low (less-than-or-equal 15th percentile) and high (greater-than-or-equal 85th percentile) body mass index (BMI) in the study population.
Subjects and method: Information on self-reported and objective weight and height was obtained from a representative sample of 3,244 adolescents, aged 15-18 years, in secondary education schools in the Autonomous Community of Madrid. We calculated the mean relative error; the correlation between subjective and objective parameters, sensitivity, specificity and predictive value positive of low and high BMIs.
Results: The mean relative errors were as follows: weight: +0.07% for males versus and 0.79% for females; height, +0.51% for males versus +0.98% for females; BMI: 0.88% for males versus 2.63% for females. The correlation between self-reported and objective BMI was 0.87 for males and 0.90 for females. The prevalence of high BMI was underestimated by 34.1% and 34.4% of females white that of low BMI was overestimated by 10.7% of males and 14.8% of females.
Conclusion: Analysis of BMI as a continuous variable, based on self-reported weight and height measurement data, entails a small margin of error. However, its use as a categorical variable involves a considerable underestimate of the prevalence of high BMI, and an smaller overestimate of the prevalence of low BMI.