Joao P Dubas
5c044e69d9
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.
47 lines
1.4 KiB
Elixir
47 lines
1.4 KiB
Elixir
defmodule Growth.Calc.Centile do
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@moduledoc """
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Calculate the value of measurement at a given z-score and fitted values of Box-Cox by age/height:
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* **power** `t:Growth.Calc.ZScore.l/0`
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* **median** `t:Growth.Calc.ZScore.m/0`
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* **coefficientof variation** `t:Growth.Calc.ZScore.s/0`
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This calculation is described in the [instructions provided by the World Health Organization](https://cdn.who.int/media/docs/default-source/child-growth/growth-reference-5-19-years/computation.pdf).
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## Examples:
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iex> measures =
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...> [
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...> [-3.0, -1.7862, 16.9392, 0.1107],
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...> [2, -1.3592, 20.4951, 0.12579],
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...> [-1.2, -1.6318, 16.049, 0.10038]
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...> ]
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iex> Enum.map(measures, &apply(Growth.Calc.Centile, :compute, &1))
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[:na, :na, 19.0]
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"""
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alias Growth.Calc.ZScore
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@spec compute(number(), ZScore.l(), ZScore.m(), ZScore.s()) :: number() | :na
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@doc """
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Compute the centile for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
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coefficient of variation (`s`).
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Returns the centile based on the z-score when -3 <= z-score <= 3; otherwise, returns `:na.`
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## Examples:
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iex> Growth.Calc.Centile.compute(0.0, -1.6318, 16.049, 0.10038)
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19.0
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"""
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def compute(z_score, l, m, s) when -3 <= z_score and z_score <= 3 do
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m * :math.pow(1 + l * s * z_score, 1 / l)
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end
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def compute(_z_score, _l, _m, _s) do
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:na
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end
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end
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