Can secondary school students' self-reported measures of height and weight be trusted? An effect size approach.

Background: Self-reported measures of height and weight are a cost-effective alternative to direct measures in large-scale studies. This study was designed to examine the accuracy of adolescent students' self-reported height and weight taking into consideration the magnitude of the differences.

Methods: Self-reported height and weight were taken from 300 secondary public schools students. Participants' actual height and weight were subsequently verified. Body mass index (BMI; kg/m2) was calculated separately from reported and from actual measures. Adolescents' whose measured BMI was above the 85th percentile were characterized as 'at risk for overweight/obese'.

Results: There was no gender effect on the discrepancy between reported and actual measures. Overall adolescents significantly underestimated their weight and BMI. Although correlation coefficients were high, eta-square (eta2) values indicate large bias for weight (0.36) and BMI (0.31). 'At risk for overweight/obese' individuals underestimated their weight and BMI to a greater extent than their 'normal weight' counterparts.

Conclusions: The magnitude of the discrepancies call into question the accuracy of self-reported weight and consequently the estimated BMI. Correlation coefficients did not provide any valuable information about the discrepancy between the self-reported and actual measures. A better understanding of the validity of self-reported height and weight could be reached if interpretation of the results is based on both statistical significance and magnitude of the differences.