diff --git a/variance_x.py b/variance_x.py
index a75e1a9..50cbc2a 100644
--- a/variance_x.py
+++ b/variance_x.py
@@ -27,6 +27,6 @@ class variance_x(algorithm):
             self.var_ = 0
         else:
             present_avg_ = self.avg_ * (self.window_length_ - 1)/self.window_length_ + value / self.window_length_
-            self.var_ = self.var_ +(value - present_avg_) * (value-self.avg_) / (present_avg_ - self.avg_)
+            self.var_ = (self.window_length_ - 1)/self.window_length_ * self.var_ + (self.window_length_ - 1) / (self.window_length_ ** 2) * ((value - self.avg_)**2)
             self.avg_ = present_avg_
         return self.var_