test(growth): add percentile test

test/growth/calc/percentile_test.exs

@@ -0,0 +1,26 @@
+defmodule Growth.Calc.PercentileTest do
+  @moduledoc false
+
+  use ExUnit.Case, async: true
+
+  doctest Growth.Calc.Percentile
+
+  alias Growth.Calc.Percentile
+
+  describe "compute/1" do
+    for {zscore, percentile} = params <- [
+          {-3, 0.0013498125},
+          {-2, 0.0227502617},
+          {-1, 0.1586553192},
+          {0, 0.5000000000},
+          {1, 0.8413446808},
+          {2, 0.9772497383},
+          {3, 0.9986501875}
+        ] do
+      @tag params: params
+      test "returns the percentile for z-score #{zscore}", %{params: {zscore, percentile}} do
+        assert_in_delta Percentile.compute(zscore), percentile, 0.0000005
+      end
+    end
+  end
+end