Table 53 Q-test for z-scores from Model 1 [BCPE(x=length (or height+0.7), df(u)=13,df(o)=6,v=1,t=2)]for weight-for-length/height for boys 142 Table 54 Goodness-of-fit summary for models BCPE(x=length(or height+0.7), df(u)=13,df(o)=6,df(v)=?,t=2)for weight-for-length/height for boys 142 Table 55 Q-test for z-scores from Model 2 [BCPE(x=length (or height+0.7), df(u)=13,df(o)=6,df(v)=1,t=2)]for weight-for-length/height for boys 145 Table 56 Observed proportions of children with measurements below the fitted centiles from Model 2,weight-for-length/height for boys 147 Table 57 Weight-for-length for boys 158 Table 58 Weight-for-height for boys 168 Table 59 Sample sizes for girls'weight-for-length/height by length interval 180 Table 60 Goodness-of-fit summary for models using the BCPE distribution with fixed v=1 and t=2 for weight-for-length/height for girls 181 Table 61 Q-test for z-scores from Model 1 [BCPE(x=length (or height+0.7), df(u)=12,df(o)=4,v=1,t=2)]for weight-for-length/height for girls 182 Table 62 Goodness-of-fit summary for models BCPE(x=length (or height+0.7), df(u)=12,df(o)=4,df(v)=?,t=2)for weight-for-length/height for girls 183 Table 63 Q-test for z-scores from Model 2 [BCPE(x=length (or height+0.7). df(u)=12,df(o)=4,df(v)=1,t=2)]for weight-for-length/height for girls 185 Table 64 Q-test for z-scores from model BCPE(x=length (or height+0.7),df(u)=12, df(o)=4,df(v)=1,t=2.13)for weight-for-length/height for girls 187 Table 65 Observed proportions of children with measurements below the fitted centiles from Model 2,weight-for-length/height for girls 188 Table 66 Weight-for-length for girls 199 Table 67 Weight-for-height for girls 209 Table 68 Longitudinal sample sizes for BMI-for-age for boys 230 Table 69 Cross-sectional sample sizes for BMI-for-age for boys 230 Table 70 Global deviance(GD)for models within the class BCPE(x=age",df(u)=9, df(o)=4,df(v)=4,t=2)for length-based BMI-for-age for boys 231 Table 71 Goodness-of-fit summary for models using the BCPE distribution with fixed v=1 and t=2 for length-based BMI-for-age for boys 231 Table 72 Q-test for z-scores from Model 1 [BCPE(x=age05,df(u)=10,df()=4, v=1,t=2)]for length-based BMI-for-age for boys 233 Table 73 Goodness-of-fit summary for models BCPE(x=age5,df()=10,df()=4. df(v)=?,t=2)for length-based BMI-for-age for boys 233 Table 74 Q-test for z-scores from Model 2 [BCPE(x=age5,df(u)=10.df()=4. df(v)=3,t=2)]for length-based BMI-for-age for boys 237 Table 75 Observed proportions of children with measurements below the fitted centiles from Model 2,length-based BMI-for-age for boys 238 Table 76 Goodness-of-fit summary for models using the BCPE distribution with fixed v=1 and t=2 for height-based BMI-for-age for boys 241 Table 77 Goodness-of-fit summary for models BCPE(x=age,df(u)=4,df(o)=3, df(v)=?,t=2)for height-based BMI-for-age for boys 242 xiv
xiv Table 53 Q-test for z-scores from Model 1 [BCPE(x=length (or height+0.7), df(µ)=13, df(σ)=6, ν=1, τ=2)] for weight-for-length/height for boys 142 Table 54 Goodness-of-fit summary for models BCPE(x=length (or height+0.7), df(µ)=13, df(σ)=6, df(ν)=?, τ=2) for weight-for-length/height for boys 142 Table 55 Q-test for z-scores from Model 2 [BCPE(x=length (or height+0.7), df(µ)=13, df(σ)=6, df(ν)=1, τ=2)] for weight-for-length/height for boys 145 Table 56 Observed proportions of children with measurements below the fitted centiles from Model 2, weight-for-length/height for boys 147 Table 57 Weight-for-length for boys 158 Table 58 Weight-for-height for boys 168 Table 59 Sample sizes for girls' weight-for-length/height by length interval 180 Table 60 Goodness-of-fit summary for models using the BCPE distribution with fixed ν=1 and τ=2 for weight-for-length/height for girls 181 Table 61 Q-test for z-scores from Model 1 [BCPE(x=length (or height+0.7), df(µ)=12, df(σ)=4, ν=1, τ=2)] for weight-for-length/height for girls 182 Table 62 Goodness-of-fit summary for models BCPE(x=length (or height+0.7), df(µ)=12, df(σ)=4, df(ν)=?, τ=2) for weight-for-length/height for girls 183 Table 63 Q-test for z-scores from Model 2 [BCPE(x=length (or height+0.7), df(µ)=12, df(σ)=4, df(ν)=1, τ=2)] for weight-for-length/height for girls 185 Table 64 Q-test for z-scores from model BCPE(x=length (or height+0.7), df(µ)=12, df(σ)=4, df(ν)=1, τ=2.13) for weight-for-length/height for girls 187 Table 65 Observed proportions of children with measurements below the fitted centiles from Model 2, weight-for-length/height for girls 188 Table 66 Weight-for-length for girls 199 Table 67 Weight-for-height for girls 209 Table 68 Longitudinal sample sizes for BMI-for-age for boys 230 Table 69 Cross-sectional sample sizes for BMI-for-age for boys 230 Table 70 Global deviance (GD) for models within the class BCPE(x=ageλ , df(µ)=9, df(σ)=4, df(ν)=4, τ=2) for length-based BMI-for-age for boys 231 Table 71 Goodness-of-fit summary for models using the BCPE distribution with fixed ν=1 and τ=2 for length-based BMI-for-age for boys 231 Table 72 Q-test for z-scores from Model 1 [BCPE(x=age0.05, df(µ)=10, df(σ)=4, ν=1, τ=2)] for length-based BMI-for-age for boys 233 Table 73 Goodness-of-fit summary for models BCPE(x=age0.05, df(µ)=10, df(σ)=4, df(ν)=?, τ=2) for length-based BMI-for-age for boys 233 Table 74 Q-test for z-scores from Model 2 [BCPE(x=age0.05, df(µ)=10, df(σ)=4, df(ν)=3, τ=2)] for length-based BMI-for-age for boys 237 Table 75 Observed proportions of children with measurements below the fitted centiles from Model 2, length-based BMI-for-age for boys 238 Table 76 Goodness-of-fit summary for models using the BCPE distribution with fixed ν=1 and τ=2 for height-based BMI-for-age for boys 241 Table 77 Goodness-of-fit summary for models BCPE(x=age, df(µ)=4, df(σ)=3, df(ν)=?, τ=2) for height-based BMI-for-age for boys 242
Table 78 Observed proportions of children with measurements below the fitted centiles from Model 1,height-based BMI-for-age for boys 244 Table 79 Q-test for z-scores from Model 1 [BCPE(x=age,df(u)=4,df(o)=3, df(v)=3,t=2)]for height-based BMI-for-age for boys 245 Table 80 Length-based BMI-for-age for boys,age in weeks 254 Table 81 Length-based BMI-for-age for boys,age in years and months 256 Table 82 Height-based BMI-for-age for boys,age in years and months 258 Table 83 Longitudinal sample sizes for BMI-for-age for girls 263 Table 84 Cross-sectional sample sizes for BMI-for-age for girls 263 Table 85 Global deviance(GD)for models within the class BCPE(x=age", df(u)=9,df(o)=4,df(v)=4,t=2)for length-based BMI-for-age for girls 264 Table 86 Goodness-of-fit summary for models using the BCPE distribution with fixed v=1 and t=2 for length-based BMI-for-age for girls 264 Table 87 Q-test for z-scores from Model 1 [BCPE(x=age5,df()=10,df()=3, v=1,t=2)]for length-based BMI-for-age for girls 266 Table 88 Goodness-of-fit summary for models BCPE(x=age05,df(u)=10,df(o)=3, df(v)=?,t=2)for length-based BMI-for-age for girls 267 Table 89 Q-test for z-scores from Model 2 [BCPE(x=age5,df(u)=10,df()=3, df(v)=3,t=2)]for length-based BMI-for-age for girls 271 Table 90 Observed proportions of children with measurements below the fitted centiles from Model 2 for length-based BMI-for-age for girls 272 Table 91 Goodness-of-fit summary for models using the BCPE distribution with fixed v=1 and t=2 for height-based BMI-for-age for girls 275 Table92 Goodness-of-fit summary for models BCPE(x=age,df(u)=4,df(o)=4, df(v)=?,t=2)for height-based BMI-for-age for girls 276 Table93 Q-test for z-scores from Model 1 [BCPE(x=age,df(u)=4,df(o)=4, df(v)=1,t=2)]for height-based BMI-for age for girls 280 Table 94 Observed proportions of children with measurements below the fitted centiles from Model 1 for height-based BMI-for-age for girls 281 Table 95 Length-based BMI-for-age for girls,age in weeks 289 Table 96 Length-based BMI-for-age for girls,age in years and months 291 Table 97 Height-based BMI-for-age for girls,age in years and months 293 XV-
- xv - Table 78 Observed proportions of children with measurements below the fitted centiles from Model 1, height-based BMI-for-age for boys 244 Table 79 Q-test for z-scores from Model 1 [BCPE(x=age, df(µ)=4, df(σ)=3, df(ν)=3, τ=2)] for height-based BMI-for-age for boys 245 Table 80 Length-based BMI-for-age for boys, age in weeks 254 Table 81 Length-based BMI-for-age for boys, age in years and months 256 Table 82 Height-based BMI-for-age for boys, age in years and months 258 Table 83 Longitudinal sample sizes for BMI-for-age for girls 263 Table 84 Cross-sectional sample sizes for BMI-for-age for girls 263 Table 85 Global deviance (GD) for models within the class BCPE(x=ageλ , df(µ)=9, df(σ)=4, df(ν)=4, τ=2) for length-based BMI-for-age for girls 264 Table 86 Goodness-of-fit summary for models using the BCPE distribution with fixed ν=1 and τ=2 for length-based BMI-for-age for girls 264 Table 87 Q-test for z-scores from Model 1 [BCPE(x=age0.05, df(µ)=10, df(σ)=3, ν=1, τ=2)] for length-based BMI-for-age for girls 266 Table 88 Goodness-of-fit summary for models BCPE(x=age0.05, df(µ)=10, df(σ)=3, df(ν)=?, τ=2) for length-based BMI-for-age for girls 267 Table 89 Q-test for z-scores from Model 2 [BCPE(x=age0.05, df(µ)=10, df(σ)=3, df(ν)=3, τ=2)] for length-based BMI-for-age for girls 271 Table 90 Observed proportions of children with measurements below the fitted centiles from Model 2 for length-based BMI-for-age for girls 272 Table 91 Goodness-of-fit summary for models using the BCPE distribution with fixed ν=1 and τ=2 for height-based BMI-for-age for girls 275 Table 92 Goodness-of-fit summary for models BCPE(x=age, df(µ)=4, df(σ)=4, df(ν)=?, τ=2) for height-based BMI-for-age for girls 276 Table 93 Q-test for z-scores from Model 1 [BCPE(x=age, df(µ)=4, df(σ)=4, df(ν)=1, τ=2)] for height-based BMI-for age for girls 280 Table 94 Observed proportions of children with measurements below the fitted centiles from Model 1 for height-based BMI-for-age for girls 281 Table 95 Length-based BMI-for-age for girls, age in weeks 289 Table 96 Length-based BMI-for-age for girls, age in years and months 291 Table 97 Height-based BMI-for-age for girls, age in years and months 293
Glossary BCPE The Box-Cox power exponential distribution. The median of the Box-Cox power exponential distribution. The approximate coefficient of variation of the Box-Cox power exponential distribution-related to the variance. The power of the Box-Cox transformation (to the normal distribution)of the Box-Cox power exponential distribution related to the skewness. The power exponential parameter of the Box-Cox power exponential distribution-related to the kurtosis. The power of the age (or length/height)transformation. Body mass index (BMD) The ratio weight (in kg)/recumbent length or standing height (in m). Box-Cox transformation A power transformation to the normal distribution. Coefficient of variation The ratio of the standard deviation to the mean. Cubic spline A piecewise third-order polynomial function that passes through a set of m (or degrees of freedom)control points;it can have a very simple form locally,yet be globally flexible and smooth. Cut-off A designated limit beyond which a subject or observation is classified according to a pre-set condition. Degrees of freedom(df) The number of control points used to fit the cubic splines. Kurtosis An attribute of a distribution describing "peakedness".A high kurtosis portrays a distribution with fat tails in contrast to a low kurtosis,which portrays a distribution with skinny tails. P-value The probability of falsely rejecting the hypothesis being tested. In this report all p-values were compared to a level of significance set to 0.05. Q-test A statistical test which combines overall and local tests assessing departures from the normal distribution with respect to median,variance,skewness and kurtosis. Skewness A statistical term used to describe a distribution's asymmetry in relation to a normal distribution. Standard deviation score (SD) See z-score Worm plots A set of detrended Q-Q plots -plots that compare the distribution of a given set of observations to the normal distribution. Z-score The deviation of an individual's value from the median value of a reference population,divided by the standard deviation of the reference population(or transformed to normal distribution). -xvi-
- xvi - Glossary BCPE The Box-Cox power exponential distribution. µ The median of the Box-Cox power exponential distribution. σ The approximate coefficient of variation of the Box-Cox power exponential distribution — related to the variance. ν The power of the Box-Cox transformation (to the normal distribution) of the Box-Cox power exponential distribution - related to the skewness. τ The power exponential parameter of the Box-Cox power exponential distribution — related to the kurtosis. λ The power of the age (or length/height) transformation. Body mass index (BMI) The ratio weight (in kg) / recumbent length or standing height (in m2 ). Box-Cox transformation A power transformation to the normal distribution. Coefficient of variation The ratio of the standard deviation to the mean. Cubic spline A piecewise third-order polynomial function that passes through a set of m (or degrees of freedom) control points; it can have a very simple form locally, yet be globally flexible and smooth. Cut-off A designated limit beyond which a subject or observation is classified according to a pre-set condition. Degrees of freedom (df) The number of control points used to fit the cubic splines. Kurtosis An attribute of a distribution describing "peakedness". A high kurtosis portrays a distribution with fat tails in contrast to a low kurtosis, which portrays a distribution with skinny tails. P-value The probability of falsely rejecting the hypothesis being tested. In this report all p-values were compared to a level of significance set to 0.05. Q-test A statistical test which combines overall and local tests assessing departures from the normal distribution with respect to median, variance, skewness and kurtosis. Skewness A statistical term used to describe a distribution's asymmetry in relation to a normal distribution. Standard deviation score (SD) See z-score. Worm plots A set of detrended Q-Q plots — plots that compare the distribution of a given set of observations to the normal distribution. Z-score The deviation of an individual's value from the median value of a reference population, divided by the standard deviation of the reference population (or transformed to normal distribution)
Executive summary In 1993 the World Health Organization (WHO)undertook a comprehensive review of the uses and interpretation of anthropometric references.The review concluded that the NCHS/WHO growth reference,which had been recommended for international use since the late 1970s,did not adequately represent early childhood growth and that new growth curves were necessary.The World Health Assembly endorsed this recommendation in 1994.In response WHO undertook the Multicentre Growth Reference Study (MGRS)between 1997 and 2003 to generate new curves for assessing the growth and development of children the world over. The MGRS combined a longitudinal follow-up from birth to 24 months and a cross-sectional survey of children aged 18 to 71 months.Primary growth data and related information were gathered from 8440 healthy breastfed infants and young children from widely diverse ethnic backgrounds and cultural settings (Brazil,Ghana,India,Norway,Oman and USA).The MGRS is unique in that it was purposely designed to produce a standard by selecting healthy children living under conditions likely to favour the achievement of their full genetic growth potential.Furthermore,the mothers of the children selected for the construction of the standards engaged in fundamental health-promoting practices,namely breastfeeding and not smoking. This report presents the first set of WHO Child Growth Standards(i.e.length/height-for-age,weight- for-age,weight-for-length,weight-for-height and body mass index(BMD)-for-age)and describes the methodical process followed in their development.The first step in this process was a consultative expert review of some 30 growth curve construction methods,including types of distributions and smoothing techniques to identify the best approach to constructing the standards.Next was the selection of a software package flexible enough to allow the comparative testing of the alternative methods used to generate the growth curves.Then the selected approach was applied systematically to search for the best models to fit the data for each indicator. The Box-Cox-power-exponential(BCPE)method,with curve smoothing by cubic splines was selected for constructing the WHO child growth curves.The BCPE accommodates various kinds of distributions,from normal to skewed or kurtotic.The age-based indicators originating at birth required a power-transformation to stretch the age scale (x-axis)as a preliminary step to fitting the curves.For each set of curves,the search for the best model specification began by examining various combinations of degrees of freedom to fit the median and variance estimator curves.When data had a non-normal distribution,degrees of freedom for parameters to model skewness and kurtosis were added to the initial model and adequacy of fit evaluated.Apart from length/height-for-age,which followed a normal distribution,the other standards required the modelling of skewness,but not kurtosis.The diagnostic tools used iteratively to detect possible model misfits and biases in the fitted curves included various tests of local and global goodness of fit,worm plots and residual plots. Patterns of differences between empirical and fitted percentiles were also examined,as were proportions of observed versus expected percentages of children with measurements below selected percentiles The methodology described above was followed to generate-for boys and girls aged 0 to 60 months -percentile and z-score curves for length/height-for-age,weight-for-age,weight-for-length,weight- for-height and BMI-for-age.The last standard is an addition to the set of indicators previously available as part of the NCHS/WHO reference.In-depth descriptions are presented of how each sex- specific standard was constructed.Also presented are comparisons of the new WHO standards with the NCHS/WHO growth reference and the CDC 2000 growth charts. To interpret differences between the WHO standards and the NCHS/WHO reference it is important to understand that they reflect differences not only in the populations used,but also in the methodologies applied to construct the two sets of growth curves.To address the significant skewness of the NCHS/WHO sample's weight-for-age and weight-for-height,separate standard deviations were xvii-
- xvii - Executive summary In 1993 the World Health Organization (WHO) undertook a comprehensive review of the uses and interpretation of anthropometric references. The review concluded that the NCHS/WHO growth reference, which had been recommended for international use since the late 1970s, did not adequately represent early childhood growth and that new growth curves were necessary. The World Health Assembly endorsed this recommendation in 1994. In response WHO undertook the Multicentre Growth Reference Study (MGRS) between 1997 and 2003 to generate new curves for assessing the growth and development of children the world over. The MGRS combined a longitudinal follow-up from birth to 24 months and a cross-sectional survey of children aged 18 to 71 months. Primary growth data and related information were gathered from 8440 healthy breastfed infants and young children from widely diverse ethnic backgrounds and cultural settings (Brazil, Ghana, India, Norway, Oman and USA). The MGRS is unique in that it was purposely designed to produce a standard by selecting healthy children living under conditions likely to favour the achievement of their full genetic growth potential. Furthermore, the mothers of the children selected for the construction of the standards engaged in fundamental health-promoting practices, namely breastfeeding and not smoking. This report presents the first set of WHO Child Growth Standards (i.e. length/height-for-age, weightfor-age, weight-for-length, weight-for-height and body mass index (BMI)-for-age) and describes the methodical process followed in their development. The first step in this process was a consultative expert review of some 30 growth curve construction methods, including types of distributions and smoothing techniques to identify the best approach to constructing the standards. Next was the selection of a software package flexible enough to allow the comparative testing of the alternative methods used to generate the growth curves. Then the selected approach was applied systematically to search for the best models to fit the data for each indicator. The Box-Cox-power-exponential (BCPE) method, with curve smoothing by cubic splines was selected for constructing the WHO child growth curves. The BCPE accommodates various kinds of distributions, from normal to skewed or kurtotic. The age-based indicators originating at birth required a power-transformation to stretch the age scale (x-axis) as a preliminary step to fitting the curves. For each set of curves, the search for the best model specification began by examining various combinations of degrees of freedom to fit the median and variance estimator curves. When data had a non-normal distribution, degrees of freedom for parameters to model skewness and kurtosis were added to the initial model and adequacy of fit evaluated. Apart from length/height-for-age, which followed a normal distribution, the other standards required the modelling of skewness, but not kurtosis. The diagnostic tools used iteratively to detect possible model misfits and biases in the fitted curves included various tests of local and global goodness of fit, worm plots and residual plots. Patterns of differences between empirical and fitted percentiles were also examined, as were proportions of observed versus expected percentages of children with measurements below selected percentiles. The methodology described above was followed to generate ─ for boys and girls aged 0 to 60 months ─ percentile and z-score curves for length/height-for-age, weight-for-age, weight-for-length, weightfor-height and BMI-for-age. The last standard is an addition to the set of indicators previously available as part of the NCHS/WHO reference. In-depth descriptions are presented of how each sexspecific standard was constructed. Also presented are comparisons of the new WHO standards with the NCHS/WHO growth reference and the CDC 2000 growth charts. To interpret differences between the WHO standards and the NCHS/WHO reference it is important to understand that they reflect differences not only in the populations used, but also in the methodologies applied to construct the two sets of growth curves. To address the significant skewness of the NCHS/WHO sample's weight-for-age and weight-for-height, separate standard deviations were
calculated for distributions below and above the median for each of the two indicators.This approach is limited in fitting skewed data,especially at the extreme tails of the distribution,since it only partially adjusts for the skewness inherent in the weight-based indicators.The WHO standards,on the other hand,employed LMS-based methods that fit skewed data adequately and generate fitted curves that follow closely the empirical data.Like the WHO standards,construction of the CDC 2000 growth charts was also based on the LMS method and,therefore,differences between this reference and the WHO standards are largely a reflection of differences in the populations on which the two sets of curves were based. Length/height-for-age.The standard for linear growth has a part based on length (length-for-age,0 to 24 months)and another on height (height-for-age,2 to 5 years).The two parts were constructed using the same model but the final curves reflect the average difference between recumbent length and standing height.By design,children between 18 and 30 months in the cross-sectional component of the MGRS had both length and height measurements taken.The average difference between the two measurements in this set of 1625 children was 0.73 cm.To fit a single model for the whole age range, 0.7 cm was therefore added to the cross-sectional height values before merging them with the longitudinal sample's length data.After the model was fitted,the median curve was shifted back downwards by 0.7 cm for ages above two years,and the coefficient of variation curve adjusted to the new median values to construct the height-for-age growth curves.The same power transformation of age was applied to stretch the age scale for each of the sexes before fitting cubic splines to generate their respective growth curves.The boys'curves required a model with higher degrees of freedom to fit both the median and coefficient of variation curves.The data for both sexes followed the normal distribution Weight-for-age.The weights of the longitudinal and cross-sectional samples were merged without any adjustments and a single model was fitted to generate one continuous set of curves constituting each sex-specific weight-for-age standard.The same power transformation was applied to both boys'and girls'age before fitting the curve construction model.The weight data for both sexes were skewed,so in specifying the model,the parameter related to skewness was fitted in addition to the median and the approximate coefficient of variation.In modelling skewness the girls'curves required more degrees of freedom to fit a curve for this parameter. Weight-for-length/height.The construction of the weight-for-length(45 to 110 cm)and weight-for- height (65 to 120 cm)standards followed a procedure similar to that applied to construct the length/height-for-age standards.That is,to fit a single model,0.7 cm was added to the cross-sectional height values,and after the model was fitted,the weight-for-length centile curves in the length interval 65.7 to 120.7 cm were shifted back by 0.7 cm to derive the weight-for-height standards corresponding to the height range 65 cm to 120 cm.The lower limit of the weight-for-length standards(45 cm)was chosen to cover up to approximately -2 SD girls'length at birth.The upper limit for the weight-for- height standards was influenced by the need to accommodate the tallest children at age 60 months,that is,120 cm is approximately +2 SD boys'height-for-age at 60 months.The overlap between the upper end of the weight-for-length standards and the lower end of the weight-for-height standards is intended to facilitate their application in severely undernourished populations and emergency settings There was no evidence that a length/height transformation similar to that described for age was required for constructing the weight-for-length/height standards.The modelling of the median and variance curves followed the procedure described for the first two standards.Results from the final model for girls'weight-for-length/height suggested the need to investigate potential improvements in the curves by modelling kurtosis.Adjustment for kurtosis,however had a negligible impact on the final centiles.Therefore,considering that modelling the fourth parameter would increase complexity in application of the standards and create inconsistency between the sexes,the final curves were generated without adjusting for kurtosis.The degrees of freedom for the median and variance curves varied between the boys'and girls'standards.The fact that the weight-for-length/height indicator combines different velocities for the two measurements involved (weight and length/height)at xviii
xviii calculated for distributions below and above the median for each of the two indicators. This approach is limited in fitting skewed data, especially at the extreme tails of the distribution, since it only partially adjusts for the skewness inherent in the weight-based indicators. The WHO standards, on the other hand, employed LMS-based methods that fit skewed data adequately and generate fitted curves that follow closely the empirical data. Like the WHO standards, construction of the CDC 2000 growth charts was also based on the LMS method and, therefore, differences between this reference and the WHO standards are largely a reflection of differences in the populations on which the two sets of curves were based. Length/height-for-age. The standard for linear growth has a part based on length (length-for-age, 0 to 24 months) and another on height (height-for-age, 2 to 5 years). The two parts were constructed using the same model but the final curves reflect the average difference between recumbent length and standing height. By design, children between 18 and 30 months in the cross-sectional component of the MGRS had both length and height measurements taken. The average difference between the two measurements in this set of 1625 children was 0.73 cm. To fit a single model for the whole age range, 0.7 cm was therefore added to the cross-sectional height values before merging them with the longitudinal sample's length data. After the model was fitted, the median curve was shifted back downwards by 0.7 cm for ages above two years, and the coefficient of variation curve adjusted to the new median values to construct the height-for-age growth curves. The same power transformation of age was applied to stretch the age scale for each of the sexes before fitting cubic splines to generate their respective growth curves. The boys' curves required a model with higher degrees of freedom to fit both the median and coefficient of variation curves. The data for both sexes followed the normal distribution. Weight-for-age. The weights of the longitudinal and cross-sectional samples were merged without any adjustments and a single model was fitted to generate one continuous set of curves constituting each sex-specific weight-for-age standard. The same power transformation was applied to both boys' and girls' age before fitting the curve construction model. The weight data for both sexes were skewed, so in specifying the model, the parameter related to skewness was fitted in addition to the median and the approximate coefficient of variation. In modelling skewness the girls' curves required more degrees of freedom to fit a curve for this parameter. Weight-for-length/height. The construction of the weight-for-length (45 to 110 cm) and weight-forheight (65 to 120 cm) standards followed a procedure similar to that applied to construct the length/height-for-age standards. That is, to fit a single model, 0.7 cm was added to the cross-sectional height values, and after the model was fitted, the weight-for-length centile curves in the length interval 65.7 to 120.7 cm were shifted back by 0.7 cm to derive the weight-for-height standards corresponding to the height range 65 cm to 120 cm. The lower limit of the weight-for-length standards (45 cm) was chosen to cover up to approximately -2 SD girls' length at birth. The upper limit for the weight-forheight standards was influenced by the need to accommodate the tallest children at age 60 months, that is, 120 cm is approximately +2 SD boys' height-for-age at 60 months. The overlap between the upper end of the weight-for-length standards and the lower end of the weight-for-height standards is intended to facilitate their application in severely undernourished populations and emergency settings. There was no evidence that a length/height transformation similar to that described for age was required for constructing the weight-for-length/height standards. The modelling of the median and variance curves followed the procedure described for the first two standards. Results from the final model for girls' weight-for-length/height suggested the need to investigate potential improvements in the curves by modelling kurtosis. Adjustment for kurtosis, however had a negligible impact on the final centiles. Therefore, considering that modelling the fourth parameter would increase complexity in application of the standards and create inconsistency between the sexes, the final curves were generated without adjusting for kurtosis. The degrees of freedom for the median and variance curves varied between the boys' and girls' standards. The fact that the weight-for-length/height indicator combines different velocities for the two measurements involved (weight and length/height) at