Standard Deviation of Anthropometric Z-scores As a Data Quality Assessment Tool Using the 2006 WHO Growth Standards: a Cross Country Analysis

Objective: Height- and weight-based anthropometric indicators are used worldwide to characterize the nutritional status of populations. Based on the 1978 WHO/National Center for Health Statistics (NCHS) growth reference, the World Health Organization has previously indicated that the standard deviation (SD) of Z-scores of these indicators is relatively constant across populations, irrespective of nutritional status. As such, the SD of Z-scores can be used as quality indicators for anthropometric data. In 2006, WHO published new growth standards. Here, we aim to assess whether the SD of height- and weight-based Z-score indicators from the 2006 WHO growth standards can still be used to assess data quality.

Methods: We examined data on children aged 0-59 months from 51 Demographic and Health Surveys (DHS) in 34 developing countries. We used 2006 growth standards to assign height-for-age Z-scores (HAZ), weight-for-age Z-scores (WAZ), weight-for-height Z-scores (WHZ) and body-mass-index-for-age Z-scores (BMIZ). We also did a stratified analysis by age group.

Findings: The SD for all four indicators were independent of their respective mean Z-scores across countries. Overall, the 5th and 95th percentiles of the SD were 1.35 and 1.95 for HAZ, 1.17 and 1.46 for WAZ, 1.08 and 1.50 for WHZ and 1.08 and 1.55 for BMIZ.

Conclusion: Our results concur with the WHO assertion that SD is in a relatively small range for each indicator irrespective of where the Z-score mean lies, and support the use of SD as a quality indicator for anthropometric data. However, the ranges of SDs for all four indicators analysed were consistently wider than those published previously by WHO.