Socioeconomic differentials in misclassification of height, weight and body mass index based on questionnaire data.

BACKGROUND: The purpose of this study was to analyse misclassification of height, weight and body mass index (BMI), derived from mail questionnaires, and its dependency on socioeconomic factors.
METHODS: A random sample of 4442 adults aged 18-84 years, living in Stockholm county, Sweden, were in 1984-1985 asked about their height and weight. A few months later 3208 of these adults participated in a health examination that included measures of height and weight. In this study we have used occupational class as the socioeconomic indicator. We have used sensitivity as a measure of misclassification of height, weight and BMI.
RESULTS: The difference in mean measured height between socioeconomic groups I and III was 2.7 cm for men and 2.0 cm for women. The mean difference in measured height between socioeconomic groups I and III was higher for men than for women in all age groups. The mean difference between self-reported and measured height was 0.6 cm for men and 0.79 cm for women. For weight, the corresponding difference was -0.74 kg for men and -1.64 kg for women. Women's BMI was more underestimated than men's (-0.85 for women, -0.40 for men). When using self-reported height and weight for calculating BMI, 81% of the men and 78% of the women were classified correctly, but only 61% of the obese men and 55% of the obese women were identified. The BMI was underestimated in all socioeconomic groups. Manual workers had a lower proportion of underweight, compared to professionals and intermediate non-manual workers, while the objective measure showed the inverse relationship. The prevalence of overweight and obesity in men was 42% higher in socioeconomic group III compared with group I with self-reported data compared with 28% when measured. Underestimation of BMI was highest among women, the obese, the elderly, and male non-manual workers and female manual workers.
CONCLUSIONS: Our study shows that socioeconomic differences in height, when using self-reported information, involve an underestimation. This means that the height differences between socioeconomic groups in Sweden may actually be higher than that reported by individuals in surveys. The socioeconomic differences in underweight tend to be underestimated for men, as well as obesity for women, when using self-reported information. The socioeconomic differences in overweight and obesity are shown to be overestimated for men.