# y2z: Convert measurements into Z-scores In growthcharts/yzy: Translate Measurements, Z-Scores and Centiles with the RIF format

 y2z R Documentation

## Convert measurements into Z-scores

### Description

Converts measurements into age- or hgt-conditional standard deviation scores (SDS, Z-score) using one or more external reference distributions.

### Usage

``````y2z(
y,
x,
refcode,
pkg = "centile",
verbose = FALSE,
dec = 3L,
rule = 1L,
...
)
``````

### Arguments

 `y` A numerical vector with measurements'. `x` A vector containing `length(y)` values of the numerical covariate (typically decimal age or height) at which conversion is desired. A scalar `x` will be expanded to `length(y)`. `refcode` A character vector with `length(y)` elements, each of which is the name of a reference. Scalar `refcode` expands to `length(y)`. Alternatively, `refcode` may be an object of class `reference`, typically produced by `import_rif()`. If `NULL` the procedure returns `NA`'s. `pkg` The package containing the references in the `R/sysdata.dta` object. The package needs to be loaded. The default `pkg = "centile"` searches in the `centile` package. `verbose` Set to `TRUE` to turn on warnings `dec` A scalar value indicating the number of decimals used to round the value. The default is 3 decimals. `rule` an integer (of length 1 or 2) describing how interpolation is to take place outside the interval [`min(x)`, `max(x)`]. If `rule` is `1` then `NA`s are returned for such points and if it is `2`, the value at the closest data extreme is used. Use, e.g., `rule = 2:1`, if the left and right side extrapolation should differ. `tail_adjust` Logical. If `TRUE` the procedure applies the WHO method for tail adjustment under the LMS distribution. We do not recommend this. The default is `FALSE`. `...` Not used.

### Value

A vector with `length(y)` elements containing the Z-scores.

### Note

The type of reference distribution is defined by the `distribution` field of the `study` attribute in the reference. The function executes transformations specified by the `tx` and `ty` fields before calculating the Z-score.

Functions `y2z()` and `z2y()` functions are the inverse of each other.

### Author(s)

Stef van Buuren, 2021

### Examples

``````# Weight SD 1 month old boy and girl of 4 KG using built-in WHO references
y <- c(4, 4)
x <- c(1/12, 1/12)
refcode <- c("who_2006_wgt_male_", "who_2006_wgt_female_")
y2z(y, x, refcode)

# using external reference, for females only
fn <- system.file("testdata/nl_2009_wgt_female_nl.txt", package = "centile")
myref <- import_rif(fn)