An improved algorithm to harmonize child overweight and obesity prevalence rates

Abstract

Background: Prevalence rates of child overweight and obesity for a group of children vary depending on the BMI reference and cut-off used. Previously we developed an algorithm to convert prevalence rates based on one reference to those based on another. Objective: To improve the algorithm by combining information on overweight and obesity prevalence. Methods: The original algorithm assumed that prevalence according to two different cut-offs A and B differed by a constant amount (Formula presented.) on the z-score scale. However the results showed that the z-score difference tended to be greater in the upper tail of the distribution and was better represented by (Formula presented.), where (Formula presented.) was a constant that varied by group. The improved algorithm uses paired prevalence rates of overweight and obesity to estimate (Formula presented.) for each group. Prevalence based on cut-off A is then transformed to a z-score, adjusted up or down according to (Formula presented.) and back-transformed, and this predicts prevalence based on cut-off B. The algorithm's performance was tested on 228 groups of children aged 6–17 years from 20 countries. Results: The revised algorithm performed much better than the original. The standard deviation (SD) of residuals, the difference between observed and predicted prevalence, was 0.8% (n = 2320 comparisons), while the SD of the difference between pairs of the original prevalence rates was 4.3%, meaning that the algorithm explained 96.7% of the baseline variance (88.2% with original algorithm). Conclusions: The improved algorithm appears to be effective at harmonizing prevalence rates of child overweight and obesity based on different references.