diff --git a/lib/growth/calc/centile.ex b/lib/growth/calc/centile.ex
index f2a3c9d..8b9dc53 100644
--- a/lib/growth/calc/centile.ex
+++ b/lib/growth/calc/centile.ex
@@ -2,9 +2,9 @@ defmodule Growth.Calc.Centile do
   @moduledoc """
   Calculate percentile for a given measurement and the fitted values of Box-Cox by age/height:
 
-  * power
-  * median
-  * coefficientof variation
+  * **power** `t:Growth.Calc.ZScore.l/0`
+  * **median** `t:Growth.Calc.ZScore.m/0`
+  * **coefficientof variation** `t:Growth.Calc.ZScore.s/0`
 
   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).
 
@@ -23,7 +23,7 @@ defmodule Growth.Calc.Centile do
 
   alias Growth.Calc.ZScore
 
-  @spec compute(number(), number(), number(), number()) :: number() | :na
+  @spec compute(number(), ZScore.l(), ZScore.m(), ZScore.s()) :: number() | :na | nil
   @doc """
   Compute the centile for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
   coefficient of variation (`s`).
@@ -36,6 +36,10 @@ defmodule Growth.Calc.Centile do
       19.0
 
   """
+  def compute(nil, _, _, _) do
+    nil
+  end
+
   def compute(y, l, m, s) do
     zscore = ZScore.raw(y, l, m, s)
 
diff --git a/lib/growth/calc/z_score.ex b/lib/growth/calc/z_score.ex
index 48a9961..36f43f5 100644
--- a/lib/growth/calc/z_score.ex
+++ b/lib/growth/calc/z_score.ex
@@ -2,9 +2,9 @@ defmodule Growth.Calc.ZScore do
   @moduledoc """
   Calculate z-score for a given measurement and the fitted values of Box-Cox by age/height:
 
-  * power
-  * median
-  * coefficientof variation
+  * **power** `t:l/0`
+  * **median** `t:m/0`
+  * **coefficientof variation** `t:s/0`
 
   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).
 
@@ -21,7 +21,20 @@ defmodule Growth.Calc.ZScore do
 
   """
 
-  @spec compute(number(), number(), number(), number()) :: number()
+  @typedoc """
+  The **power** value in Box-Cox transformation.
+  """
+  @type l :: number()
+  @typedoc """
+  The **median** value in Box-Cox transformation.
+  """
+  @type m :: number()
+  @typedoc """
+  The **coefficient of variation** in Box-Cox transformation.
+  """
+  @type s :: number()
+
+  @spec compute(number(), l(), m(), s()) :: Growth.measure()
   @doc """
   Compute the adjusted z-score for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
   coefficient of variation (`s`).
@@ -36,13 +49,18 @@ defmodule Growth.Calc.ZScore do
       1.4698319520484722
 
   """
+
+  def compute(nil, _, _, _) do
+    nil
+  end
+
   def compute(y, l, m, s) do
     y
     |> raw(l, m, s)
     |> adjust(y, l, m, s)
   end
 
-  @spec raw(number(), number(), number(), number()) :: number()
+  @spec raw(number(), l(), m(), s()) :: number()
   @doc """
   Compute the raw z-score for a given measurement (`y`) and the Box-Cox values, power (`l`), median (`m`), and
   coefficient of variation (`s`).
@@ -61,7 +79,7 @@ defmodule Growth.Calc.ZScore do
     (:math.pow(y / m, l) - 1) / (s * l)
   end
 
-  @spec adjust(number(), number(), number(), number(), number()) :: number()
+  @spec adjust(number(), number(), l(), m(), s()) :: number()
   @doc """
   Adjust the raw z-score considering that extreme values, beyond -3 and 3 standard deviation, must be handled differently.
   """