overlapping ages likely explains the slight wiggle in the final WHO standards(for both boys and girls) as also observed in other references. Body mass index-for-age.Body mass index is the ratio weight (in kg)/recumbent length or standing height(in m2).To address the difference between length and height,the approach used for constructing the BMI-for-age standards was different from that described for length/height-for-age.Because BMI is a ratio with squared length or height in the denominator,adding 0.7 cm to the height values and back- transforming them after fitting was not feasible.The solution adopted was to construct the standards for the younger and the older children separately based on two sets of data with an overlapping range of ages below and above 24 months.To construct the BMI-for-age standard based on length (0 to 2 years).the longitudinal sample's length data and the cross-sectional sample's height data (18 to 30 months)were combined after adding 0.7 cm to the height values.Analogously,to construct the standard from 2 to 5 years,the cross-sectional sample's height plus the longitudinal sample's length data (18 to 24 months)were combined after subtracting 0.7 cm from the length values.Thus,a common set of data from 18 to 30 months was used to generate the BMI standards for the younger and the older children.The resulting disjunction between the two standards thus in essence reflects the 0.7 cm difference between length and height.This does not mean,however,that a child at a specific age will have the same length-and height-based BMI-for-age z-score as this is mathematically impossible given the nature of the BMI ratio. An age power transformation as described for the other age-based standards was required before constructing the length-based BMI-for-age curves.No such transformation was necessary for the height-based BMI-for-age.The WHO length-and height-based BMI-for-age standards do not overlap, i.e.the length-based interval ends at 730 days and the height-based interval starts at 731 days.Cubic spline fitting was achieved with variable degrees of freedom for the length-versus height-based standards,and also for the boys'versus girls'final curves. Technical aspects of the standards.The method used to construct the WHO standards generally relied on the Box-Cox power exponential distribution and the final selected models simplified to the LMS model.As a result,the computation of percentiles and z-scores for these standards uses formulae based on the LMS method.However,a restriction was imposed on all indicators to enable the derivation of percentiles only within the interval corresponding to z-scores between -3 and 3.The underlying reasoning is that percentiles beyond +3 SD are invariant to changes in equivalent z-scores. The loss accruing to this restriction is small since the inclusion range corresponds to the 0.135th to 99.865th percentiles The weight-based indicators presented right-skewed distributions.When modelled correctly,right skewness has the effect of making distances between positive z-scores increase progressively the farther away they are from the median,while distances between negative z-scores decrease progressively.The LMS method fits skewed data adequately by using a Box-Cox normal distribution, which follows the empirical data closely.The drawback,however,is that the outer tails of the distribution are highly affected by extreme data points even if only very few.A restricted application of the LMS method was thus used for the construction of the WHO weight-based indicators,limiting the Box-Cox normal distribution to the interval corresponding to z-scores where empirical data were available (i.e.between-3 SD and 3 SD).Beyond these limits,the standard deviation at each age (or length/height)was fixed to the distance between +2 SD and +3 SD,respectively.This approach avoids making assumptions about the distribution of data beyond the limits of the observed values. Epidemiological aspects of the standards.As expected,there are notable differences with the NCHS/WHO reference that vary by age,sex,anthropometric measure and specific percentile or z-score curve.Differences are particularly important in infancy.Stunting will be greater throughout childhood when assessed using the new WHO standards compared to the NCHS/WHO reference.The growth pattern of breastfed infants will result in a substantial increase in rates of underweight during the first half of infancy and a decrease thereafter.For wasting,the main difference is during infancy XiX-- xix - overlapping ages likely explains the slight wiggle in the final WHO standards (for both boys and girls) as also observed in other references. Body mass index-for-age. Body mass index is the ratio weight (in kg)/recumbent length or standing height (in m2 ). To address the difference between length and height, the approach used for constructing the BMI-for-age standards was different from that described for length/height-for-age. Because BMI is a ratio with squared length or height in the denominator, adding 0.7 cm to the height values and back￾transforming them after fitting was not feasible. The solution adopted was to construct the standards for the younger and the older children separately based on two sets of data with an overlapping range of ages below and above 24 months. To construct the BMI-for-age standard based on length (0 to 2 years), the longitudinal sample's length data and the cross-sectional sample's height data (18 to 30 months) were combined after adding 0.7 cm to the height values. Analogously, to construct the standard from 2 to 5 years, the cross-sectional sample's height plus the longitudinal sample's length data (18 to 24 months) were combined after subtracting 0.7 cm from the length values. Thus, a common set of data from 18 to 30 months was used to generate the BMI standards for the younger and the older children. The resulting disjunction between the two standards thus in essence reflects the 0.7 cm difference between length and height. This does not mean, however, that a child at a specific age will have the same length- and height-based BMI-for-age z-score as this is mathematically impossible given the nature of the BMI ratio. An age power transformation as described for the other age-based standards was required before constructing the length-based BMI-for-age curves. No such transformation was necessary for the height-based BMI-for-age. The WHO length- and height-based BMI-for-age standards do not overlap, i.e. the length-based interval ends at 730 days and the height-based interval starts at 731 days. Cubic spline fitting was achieved with variable degrees of freedom for the length- versus height-based standards, and also for the boys' versus girls' final curves. Technical aspects of the standards. The method used to construct the WHO standards generally relied on the Box-Cox power exponential distribution and the final selected models simplified to the LMS model. As a result, the computation of percentiles and z-scores for these standards uses formulae based on the LMS method. However, a restriction was imposed on all indicators to enable the derivation of percentiles only within the interval corresponding to z-scores between -3 and 3. The underlying reasoning is that percentiles beyond ±3 SD are invariant to changes in equivalent z-scores. The loss accruing to this restriction is small since the inclusion range corresponds to the 0.135th to 99.865th percentiles. The weight-based indicators presented right-skewed distributions. When modelled correctly, right skewness has the effect of making distances between positive z-scores increase progressively the farther away they are from the median, while distances between negative z-scores decrease progressively. The LMS method fits skewed data adequately by using a Box-Cox normal distribution, which follows the empirical data closely. The drawback, however, is that the outer tails of the distribution are highly affected by extreme data points even if only very few. A restricted application of the LMS method was thus used for the construction of the WHO weight-based indicators, limiting the Box-Cox normal distribution to the interval corresponding to z-scores where empirical data were available (i.e. between -3 SD and 3 SD). Beyond these limits, the standard deviation at each age (or length/height) was fixed to the distance between ±2 SD and ±3 SD, respectively. This approach avoids making assumptions about the distribution of data beyond the limits of the observed values. Epidemiological aspects of the standards. As expected, there are notable differences with the NCHS/WHO reference that vary by age, sex, anthropometric measure and specific percentile or z-score curve. Differences are particularly important in infancy. Stunting will be greater throughout childhood when assessed using the new WHO standards compared to the NCHS/WHO reference. The growth pattern of breastfed infants will result in a substantial increase in rates of underweight during the first half of infancy and a decrease thereafter. For wasting, the main difference is during infancy