The `daz()`

function calculated the
"Development for Age Z-score".
The DAZ represents a child's D-score after adjusting
for age by an external age-conditional reference.
The `zad()`

is the inverse of `daz()`

: Given age and
the Z-score, it finds the raw D-score.

```
daz(d, x, reference = get_reference(), dec = 3)
zad(z, x, reference = get_reference(), dec = 2)
```

- d
Vector of D-scores

- x
Vector of ages (decimal age)

- reference
A

`data.frame`

with the LMS reference values. The default uses the`get_reference()`

function. This selects a subset of rows from the`builtin_references`

.- dec
The number of decimals (default

`dec = 3`

).- z
Vector of standard deviation scores (DAZ)

Unnamed numeric vector with Z-scores of length `length(d)`

.

Unnamed numeric vector with D-scores of length `length(z)`

.

Note 1: The Box-Cox Cole and Green (BCCG) and Box-Cox t (BCT)
distributions model only positive D-score values. To increase
robustness, the `daz()`

and `zad()`

functions will round up any
D-scores lower than 1.0 to 1.0.

Note 2: The `daz()`

and `zad()`

function call modified version of the
`pBCT()`

and `qBCT()`

functions from `gamlss`

for better handling
of `NA`

's and rounding.

Cole TJ, Green PJ (1992). Smoothing reference centile curves: The LMS method and penalized likelihood. Statistics in Medicine, 11(10), 1305-1319.

```
# using GSED Phase 1 reference
daz(d = c(35, 50), x = c(0.5, 1.0))
#> [1] 0.788 0.587
# using Dutch reference
daz(d = c(35, 50), x = c(0.5, 1.0), reference = get_reference("gcdg"))
#> [1] -0.425 0.299
# using Dutch reference
daz(d = c(35, 50), x = c(0.5, 1.0), reference = get_reference("dutch"))
#> [1] -0.091 0.357
# population median at ages 0.5, 1 and 2 years, phase1 reference
zad(z = rep(0, 3), x = c(0.5, 1, 2))
#> [1] 32.28 47.93 64.30
# population median at ages 0.5, 1 and 2 years, gcdg reference
zad(z = rep(0, 3), x = c(0.5, 1, 2), reference = get_reference("gcdg"))
#> [1] 36.32 49.11 62.67
# population median at ages 0.5, 1 and 2 years, dutch reference
zad(z = rep(0, 3), x = c(0.5, 1, 2), reference = get_reference("dutch"))
#> [1] 35.27 48.91 63.77
# percentiles of D-score reference
g <- expand.grid(age = seq(0.1, 2, 0.1), p = c(0.1, 0.5, 0.9))
d <- zad(z = qnorm(g$p), x = g$age)
matplot(
x = matrix(g$age, ncol = 3), y = matrix(d, ncol = 3), type = "l",
lty = 1, col = "blue", xlab = "Age (years)", ylab = "D-score"
)
```