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fix(growth): use centile and percentile correctly

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1. The calculation made in `Grownth.Calc.Centile.compute/4` converts a
   given z-score into the expected measurement value.
   This can be used to get specific values for key z-scores, such as,
   -3, -2, 2 and 3.
2. To calculate the percentile of a given z-score the system uses the
   cumulative distribution function.
This commit is contained in:
João Paulo Dubas
2024-08-28 13:03:45 +00:00
parent 824a70452f
commit 5c044e69d9
3 changed files with 56 additions and 30 deletions
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22
lib/growth/calc/centile.ex
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@@ -1,6 +1,6 @@
defmodule Growth.Calc.Centile do
@moduledoc """
Calculate percentile for a given measurement and the fitted values of Box-Cox by age/height:
Calculate the value of measurement at a given z-score and fitted values of Box-Cox by age/height:
* **power** `t:Growth.Calc.ZScore.l/0`
* **median** `t:Growth.Calc.ZScore.m/0`
@@ -12,9 +12,9 @@ defmodule Growth.Calc.Centile do
iex> measures =
...> [
...> [30, -1.7862, 16.9392, 0.1107],
...> [14, -1.3592, 20.4951, 0.12579],
...> [19, -1.6318, 16.049, 0.10038]
...> [-3.0, -1.7862, 16.9392, 0.1107],
...> [2, -1.3592, 20.4951, 0.12579],
...> [-1.2, -1.6318, 16.049, 0.10038]
...> ]
iex> Enum.map(measures, &apply(Growth.Calc.Centile, :compute, &1))
[:na, :na, 19.0]
@@ -32,17 +32,15 @@ defmodule Growth.Calc.Centile do
## Examples:
iex> Growth.Calc.Centile.compute(19, -1.6318, 16.049, 0.10038)
iex> Growth.Calc.Centile.compute(0.0, -1.6318, 16.049, 0.10038)
19.0
"""
def compute(y, l, m, s) do
zscore = ZScore.raw(y, l, m, s)
def compute(z_score, l, m, s) when -3 <= z_score and z_score <= 3 do
m * :math.pow(1 + l * s * z_score, 1 / l)
end
if -3 <= zscore and zscore <= 3 do
m * :math.pow(1 + l * s * zscore, 1 / l)
else
:na
end
def compute(_z_score, _l, _m, _s) do
:na
end
end