diff --git a/lib/bp.js b/lib/bp.js
index ea62434..ee22825 100644
--- a/lib/bp.js
+++ b/lib/bp.js
@@ -64,7 +64,7 @@ bp.Statistics.calculateStandardDeviation = function(sample, mean) {
   sample.forEach(function(x) {
     deviation += Math.pow(x - mean, 2);
   });
-  deviation = deviation / (sample.length -1);
+  deviation = deviation / sample.length;
   deviation = Math.sqrt(deviation);
   return deviation;
 };