diff --git a/src/Phpml/NeuralNetwork/Training/Backpropagation.php b/src/Phpml/NeuralNetwork/Training/Backpropagation.php
index e6691e2..3f80bf5 100644
--- a/src/Phpml/NeuralNetwork/Training/Backpropagation.php
+++ b/src/Phpml/NeuralNetwork/Training/Backpropagation.php
@@ -21,6 +21,11 @@ class Backpropagation implements Training
      */
     private $theta;
 
+    /**
+     * @var array
+     */
+    private $sigmas;
+
     /**
      * @param Network $network
      * @param int     $theta
@@ -71,20 +76,22 @@ class Backpropagation implements Training
         return $resultsWithinError;
     }
 
+    /**
+     * @param array $sample
+     * @param array $target
+     */
     private function trainSample(array $sample, array $target)
     {
         $this->network->setInput($sample)->getOutput();
+        $this->sigmas = [];
 
-        $sigmas = [];
         $layers = $this->network->getLayers();
         $layersNumber = count($layers);
 
         for ($i = $layersNumber; $i > 1; --$i) {
             foreach ($layers[$i - 1]->getNodes() as $key => $neuron) {
                 if ($neuron instanceof Neuron) {
-                    $neuronOutput = $neuron->getOutput();
-                    $sigma = $neuronOutput * (1 - $neuronOutput) * ($i == $layersNumber ? ($target[$key] - $neuronOutput) : $this->getPrevSigma($sigmas, $neuron));
-                    $sigmas[] = new Sigma($neuron, $sigma);
+                    $sigma = $this->getSigma($neuron, $target, $key, $i == $layersNumber);
                     foreach ($neuron->getSynapses() as $synapse) {
                         $synapse->changeWeight($this->theta * $sigma * $synapse->getNode()->getOutput());
                     }
@@ -94,21 +101,40 @@ class Backpropagation implements Training
     }
 
     /**
-     * @param Sigma[] $sigmas
-     * @param Neuron  $forNeuron
+     * @param Neuron $neuron
+     * @param array  $target
+     * @param int    $key
+     * @param bool   $lastLayer
+     *
+     * @return float
+     */
+    private function getSigma(Neuron $neuron, array $target, int $key, bool $lastLayer): float
+    {
+        $neuronOutput = $neuron->getOutput();
+        $sigma = $neuronOutput * (1 - $neuronOutput);
+
+        if ($lastLayer) {
+            $sigma *= ($target[$key] - $neuronOutput);
+        } else {
+            $sigma *= $this->getPrevSigma($neuron);
+        }
+
+        $this->sigmas[] = new Sigma($neuron, $sigma);
+
+        return $sigma;
+    }
+
+    /**
+     * @param Neuron $neuron
      * 
      * @return float
      */
-    private function getPrevSigma(array $sigmas, Neuron $forNeuron): float
+    private function getPrevSigma(Neuron $neuron): float
     {
         $sigma = 0.0;
 
-        foreach ($sigmas as $neuronSigma) {
-            foreach ($neuronSigma->getNeuron()->getSynapses() as $synapse) {
-                if ($synapse->getNode() == $forNeuron) {
-                    $sigma += $synapse->getWeight() * $neuronSigma->getSigma();
-                }
-            }
+        foreach ($this->sigmas as $neuronSigma) {
+            $sigma += $neuronSigma->getSigmaForNeuron($neuron);
         }
 
         return $sigma;
diff --git a/src/Phpml/NeuralNetwork/Training/Backpropagation/Sigma.php b/src/Phpml/NeuralNetwork/Training/Backpropagation/Sigma.php
index 8ce397b..8520354 100644
--- a/src/Phpml/NeuralNetwork/Training/Backpropagation/Sigma.php
+++ b/src/Phpml/NeuralNetwork/Training/Backpropagation/Sigma.php
@@ -43,4 +43,22 @@ class Sigma
     {
         return $this->sigma;
     }
+
+    /**
+     * @param Neuron $neuron
+     *
+     * @return float
+     */
+    public function getSigmaForNeuron(Neuron $neuron): float
+    {
+        $sigma = 0.0;
+
+        foreach ($this->neuron->getSynapses() as $synapse) {
+            if ($synapse->getNode() == $neuron) {
+                $sigma += $synapse->getWeight() * $this->getSigma();
+            }
+        }
+
+        return $sigma;
+    }
 }
diff --git a/tests/Phpml/NeuralNetwork/Training/BackpropagationTest.php b/tests/Phpml/NeuralNetwork/Training/BackpropagationTest.php
index a44c1d5..265d936 100644
--- a/tests/Phpml/NeuralNetwork/Training/BackpropagationTest.php
+++ b/tests/Phpml/NeuralNetwork/Training/BackpropagationTest.php
@@ -18,7 +18,7 @@ class BackpropagationTest extends \PHPUnit_Framework_TestCase
             [[1, 0], [0, 1], [1, 1], [0, 0]],
             [[1], [1], [0], [0]],
             $desiredError = 0.2,
-            10000
+            30000
         );
 
         $this->assertEquals(0, $network->setInput([1, 1])->getOutput()[0], '', $desiredError);