A unified pattern standard score method for construction of age-related weight-for-height

Analysis of data from the Shiraz (Iran) growth study requires to take age into account when assessing weight-for-height of school children aged 6-12. A unified structure standard score method for construction of weight-for-height is designed to overcome the age dependence problem. This method first applies Healy-Rasbash-Yang (HRY) distribution-free estimation method of age-related centiles to calculate log (weight) and height-for-age centiles, and convert them to their standard scores. These scores are age and sex independent, and are called weight index (WI) and height index (HI), respectively. Then WI is regressed on HI and age-adjusted centiles of weight-for-height are derived at each age of weight and height to their respective zeta-scores. Finally, weight-for-height centiles are drawn for each age-sex category of children. This zeta-score method is shown to be efficient, simple, and stable, and may generally be applied to other data.