sitar (version 1.0.1)

lms2z: Convert to/from measurement from/to z-score with growth reference

Description

A function to convert between measurements and z-scores using a growth reference previously fitted by the LMS method.

Usage

lms2z(x, y, sex, data = parent.frame(), measure, ref, toz = TRUE)

Arguments

x

vector of ages

y

vector of either measurements or z-scores, depending on the value of toz.

sex

two-level factor with level 1 male and level 2 female.

data

optional data frame containing x, y and sex.

measure

character indicating the name of the measurement, depending on the choice of ref (see e.g. references uk90, who06 and ukwhopt).

ref

character indicating the name of the growth reference, available as a data object.

toz

logical set to TRUE for conversion from measurement to z-score, or FALSE for the reverse.

Value

• A vector of the same length as y containing the transformed values.

Details

Vectors of L, M and S corresponding to x and sex are extracted and passed to either cLMS or zLMS, depending on toz.

See Also

z2cent. The LMS method can be fitted to data using the package gamlss with the BCCG family, where nu (originally lambda), mu and sigma correspond to L, M and S respectively.

Examples

## convert girls' heights data to UK 90 z-scores
data(heights)
data(uk90)
lms2z(age, height, sex = 2, heights, measure = 'ht', ref = 'uk90')

## construct table of boys weight centiles for WHO standard on 3-month age grid
data(who06)
zs <- -4:4*2/3 # z-scores for centiles
ages <- 0:12/4 # 3-month ages
v <- as.data.frame(lapply(as.list(zs), function(z) {
  v <- lms2z(ages, z, sex = 1, measure = 'wt', ref = 'who06', toz = FALSE)
}))
names(v) <- z2cent(zs)
rownames(v) <- ages
round(v, 2)