diff --git a/Math/Stochastic/Distribution/ZTest.php b/Math/Stochastic/Distribution/ZTest.php
index c7f4464a0..271d48d44 100644
--- a/Math/Stochastic/Distribution/ZTest.php
+++ b/Math/Stochastic/Distribution/ZTest.php
@@ -61,7 +61,7 @@ final class ZTest
             }
         }
 
-        return $z > -$key && $z < $key;
+        return $z > -$zSignificance && $z < $zSignificance;
     }
 
     /**