who_z: Sex- and Age- (or Length-) Standardized Weight,...
In cdcanthro: Standardized Metrics Based on the CDC and WHO Growth Charts

View source: R/who_z.R

who_z

R Documentation

Sex- and Age- (or Length-) Standardized Weight, Length/Height, BMI, and Head Circumference Metrics from the WHO Growth Charts

Description

Calculates z-scores and biologically-implausible-value (BIV) flags for weight-for-age, length/height-for-age, BMI-for-age, head-circumference-for-age, and weight-for-length, based on the WHO Child Growth Standards. The original data are returned together with the calculated metrics.

This function is included in the cdcanthro package because CDC recommends using (1) the CDC Growth Charts for children with ages (months) >=24 and < 240, and (2) the WHO Standards for children < 24 months of age. The use of these different charts, along with the change from recumbent length to standing height at age 2.0 y, creates discontinuities at age 2.0 years (730 days). https://www.cdc.gov/growth-chart-training/hcp/overview/recommended.html

Because this function is written using data.table it's much faster than some other packages that calculate WHO z-scores.

Usage

who_z(data,
          agedays = agedays,
          wt = wt,
          lenhei = lenhei,
          bmi = bmi,
          headc = headc,
          sex_code = 1:2)

Arguments

`data`	a `data.frame`, `data.table`, or tibble (`tbl_df`) containing the input data. The class of the returned object matches the class of `data`. Must contain a variable named `sex` or `SEX` (but not both).
`agedays`	age in completed days. Note that all arguments can be unquoted (e.g., agedays = days) or quoted(agedays = 'days'). If age is given as completed number of months (e.g., NHANES), add 0.5 months, multiply by 30.4375 to convert to agedays, and round to the nearest day.
`wt`	weight in kg.
`lenhei`	recumbent length (ages <= 730 days) or standing height (ages > 730 days), in cm. Note that there is an ~0.7 cm difference in M (median) between 730 and 731 days in the reference data, when the measurement changes from recumbent length to standing height. The reference data contain `lenhei` to the nearest 0.1 cm.
`bmi`	body mass index, kg/m^2. If not supplied but both `wt` and `lenhei` are, BMI is calculated as `wt / (lenhei/100)^2`.
`headc`	head circumference in cm.
`sex_code`	Numeric, single-digit coding for sex if other than 1 for boys and 2 for girls. Optional.

Details

Column names can be given with or without quotation marks. Naming the arguments rather than relying on position is recommended, e.g. who_z(d, agedays = age, wt = weight); otherwise a value can silently land in the wrong slot. Any absent measurement is skipped, and its z-score is not returned.

The function expects the child's sex to be named sex; either upper- or lower-case is fine. You cannot have both SEX and sex in your data. Values can be coded as boys/girls, b/g, male/female, m/f, or 1/2. Character values can be upper- or lower-case, and only the first character is considered. Be aware that a code of 'missing' will be interpreted as a boy.

If sex is numeric and not coded as 1/2, specify this in the optional sex_code argument; the code for boys is the first number. For example, 0/1 coding would be specified as sex_code = 0:1 or sex_code = c(0,1).

At least one of wt, lenhei, bmi or headc must be present. Metrics that cannot be calculated from the supplied variables are silently omitted from the output. If a supplied column name doesn't exist in data (a likely typo), a warning is issued.

Age in the WHO reference data goes from 0 to 1856 days (1856/365.25 = 5.08 y), but calculations in who_z go up to 1826 days (4.993 y). Weight-for-length Z is calculated only for children <= 730 days of age with a length between 45 and 110 cm.

Value

A data.table, tibble, or dataframe containing the original data plus the calculated metrics.

WHO Z-scores:

who_waz: weight-for-sex/age
who_lhaz: length (or height) for sex/age
who_wflz: weight-for-length for sex
who_bmiz: BMI-for-sex/age
who_headcz: head circumference for sex/age

BIV flags, following the anthro package. Each 'f' (flag) is 1 if the z-score is outside the WHO plausibility range, 0 if inside, and NA if the z-score is NA:

fwaz: 1 if who_waz < -6 or > 5
flhaz: 1 if who_lhaz < -6 or > 6
fwflz: 1 if who_wflz < -5 or > 5
fbmiz: 1 if who_bmiz < -5 or > 5
fheadcz: 1 if who_headcz < -5 or > 5

Note that there doesn't appear to be any publication that justifies these cutpoints. While values outside these ranges are outliers, the measurement may be correct.

Author(s)

David Freedman DavidSFreedman@gmail.com

References

World Health Organization, Department of Nutrition for Health and Development. WHO Child Growth Standards: Length/height-for-age, weight-for-age, weight-for-length, weight-for-height, and body mass index-for-age. Methods and Development. Geneva; 2006. https://apps.who.int/iris/handle/10665/43413

The reference WHO LMS values for weight-for-age are in the 'Expanded Tables' files at https://www.who.int/tools/child-growth-standards/standards/weight-for-age. Reference values for the other measures are in the left column.

Examples

NHanes
d <- data.table::copy(NHanes)
d[, let(lenhei=data.table::fifelse(agedays<=730,length,ht))]
# lenhei is a composite of length and ht:
# it is length for 0 to <=730 days, but height for >730 days
d <- d[agedays <= 1826]
out <- who_z(d, agedays, wt, lenhei, bmi, headc)
head(out)

# verify that the function works for head circumference (small N)
temp <- NHanes[!is.na(headc)]
temp[, let(lenhei=data.table::fifelse(agedays<=730,length,ht))]
out <- who_z(temp, agedays, wt, lenhei, bmi, headc)
head(out)

cdcanthro documentation built on Oct. 31, 2026, 1:07 a.m.