How valid are web-based self-reports of weight?

Background: many studies rely on self-reported anthropometric data. while paper-based self-reports have been the standard collection mode,the number of studies collecting self-reported data via the web is increasing rapidly. although numerous studies have shown good agreement between self-reported and measured weight using paper-based questionnaires,the validity of using the web to inquire about weight is unknown. objective: the objective of this study was to validate web-based self-reports of bodyweight compared to weight measured at the study center. methods: the validity of weight self-reported via the web was assessed by comparing self-reports against measurements of weight in a convenience sample of 149 individuals (77.2% women,115/149),aged 20-65 years. study participants self-reported their weight via a web-based questionnaire and thereafter had their weight measured in the research center. results: the spearman correlation coefficient between self-reported and measured weight was 0.98 (p<.001). the mean difference between self-reported and measured weight was -1.2 (sd 2.6) kg. there was a statistically significant difference between self-reported and measured weight with the self-reported being lower (p<.001). subjects with a body mass index (bmi) =25 kg/m2,and subjects =30 years of age,under-reported their weight statistically significantly more than subjects with a bmi <25 kg/m2,and subjects <30 years of age,respectively. conclusions: our results show that self-reported weight via the web can be a valid method of data collection.