Effects of categorization and self-report bias on estimates of the association between obesity and mortality.

PURPOSE

The health consequences of obesity are often assessed using categorical, self-reported data on body mass index (BMI). This article investigates the combined effects of categorization and self-report bias on the estimated association between obesity and mortality.

METHODS

We used the National Health and Nutrition Examination Survey (1988-2008) linked to death records through 2011. Cox models and age-standardized death rates were used to evaluate the effects of categorization and self-report bias on the mortality risks and percent of deaths attributable to obesity.

RESULTS

Despite a correlation between measured and self-reported BMI of 0.96, self-reports miscategorized 20% of adults. Hazard ratios using self-reports were overstated for the obese 1 (BMI, 30-35 kg/m(2)) and obese 2 (BMI ≥ 35 kg/m(2)) categories. The bias was much smaller using a continuous measure of BMI. In contrast, the percent of deaths attributable to excess weight was lower using self-reported versus measured data because self-reports led to systematic downward bias in the BMI distribution.

CONCLUSIONS

Categorization of BMI and self-report bias combine to produce substantial error in the estimated hazard ratios and percent of deaths attributable to obesity. Future studies should use caution when estimating the association between obesity and mortality using categorical self-reported data.