Returns the age-interpolated median of the phase1 references
based on LF & SF in GSED-BGD, GSED-PAK, GSED-TZA.

## Arguments

- t
Decimal age, numeric vector

## Value

A vector of length `length(t)`

with the median of the GCDG references.

## Details

The interpolation is done in two rounds. First round: Calculate D-scores
using .gcdg prior-mean, calculate reference, estimate round 1 parameters
used in this function. Round 2: Calculate D-score using round 1 estimates as
the prior mean (most differences are within 0.1 D-score points),
recalculate references, estimate round 2 parameters used in this function.

Round 1:
Count model: <= 9MN: 21.3449 + 26.4916 t + 7.0251(t + 0.2)
Count model: > 9Mn & <= 3.5 YR: 14.69947 - 12.18636 t + 69.11675(t + 0.92)
Linear model: > 3.5 YRS: 61.40956 + 3.80904 t

Round 2:
Count model: < 9MND: 20.5883 + 27.3376 t + 6.4254(t + 0.2)
Count model: > 9MND & < 3.5 YR: 14.63748 - 12.11774 t + 69.05463(t + 0.92)
Linear model: > 3.5 YRS: 61.37967 + 3.83513 t

The working range is 0-3.5 years. After the age of 3.5 years, the function
will increase at an arbitrary rate of 3.8 D-score points per year.
This function is intended to be called when `key = "gsed2212"`

,
`key = "gsed2208"`

or `key = "293_0"`

.

## Note

Internal function. Called by `dscore()`

## Author

Stef van Buuren, on behalf of GSED project

## Examples

```
dscore:::count_mu_phase1(0:5)
#> [1] 10.24702 47.56581 64.39981 72.61921 76.72019 80.55532
```