diff --git a/lib/growth/calc/centile.ex b/lib/growth/calc/centile.ex
index 3acd78c..f2a3c9d 100644
--- a/lib/growth/calc/centile.ex
+++ b/lib/growth/calc/centile.ex
@@ -23,11 +23,13 @@ defmodule Growth.Calc.Centile do
 
   alias Growth.Calc.ZScore
 
-  @spec compute(number(), number(), number(), number()) :: number()
+  @spec compute(number(), number(), number(), number()) :: number() | :na
   @doc """
   Compute the centile for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
   coefficient of variation (`s`).
 
+  Returns the centile based on the z-score when -3 <= z-score <= 3; otherwise, returns `:na.`
+
   ## Examples:
 
       iex> Growth.Calc.Centile.compute(19, -1.6318, 16.049, 0.10038)