diff --git a/src/Phpml/Exception/NormalizerException.php b/src/Phpml/Exception/NormalizerException.php
index 9f88f0c..5c7ced1 100644
--- a/src/Phpml/Exception/NormalizerException.php
+++ b/src/Phpml/Exception/NormalizerException.php
@@ -13,4 +13,13 @@ class NormalizerException extends \Exception
     {
         return new self('Unknown norm supplied.');
     }
+
+    /**
+     * @return NormalizerException
+     */
+    public static function fitNotAllowed()
+    {
+        return new self('Fit is not allowed for this preprocessor.');
+    }
+
 }
diff --git a/src/Phpml/Exception/PreprocessorException.php b/src/Phpml/Exception/PreprocessorException.php
deleted file mode 100644
index 15e3975..0000000
--- a/src/Phpml/Exception/PreprocessorException.php
+++ /dev/null
@@ -1,17 +0,0 @@
-<?php
-declare(strict_types = 1);
-
-namespace Phpml\Exception;
-
-class PreprocessorException extends \Exception
-{
-
-    /**
-     * @return PreprocessorException
-     */
-    public static function fitNotAllowed()
-    {
-        return new self('Fit is not allowed for this preprocessor.');
-    }
-
-}
\ No newline at end of file
diff --git a/src/Phpml/Pipeline.php b/src/Phpml/Pipeline.php
index f230db3..b55abca 100644
--- a/src/Phpml/Pipeline.php
+++ b/src/Phpml/Pipeline.php
@@ -67,6 +67,7 @@ class Pipeline implements Estimator
      */
     public function train(array $samples, array $targets)
     {
+        $this->fitTransformers($samples);
         $this->transformSamples($samples);
         $this->estimator->train($samples, $targets);
     }
@@ -83,6 +84,16 @@ class Pipeline implements Estimator
         return $this->estimator->predict($samples);
     }
 
+    /**
+     * @param array $samples
+     */
+    private function fitTransformers(array &$samples)
+    {
+        foreach ($this->transformers as $transformer) {
+            $transformer->fit($samples);
+        }
+    }
+
     /**
      * @param array $samples
      */
diff --git a/src/Phpml/Preprocessing/Imputer.php b/src/Phpml/Preprocessing/Imputer.php
index fdbfaf6..424efa4 100644
--- a/src/Phpml/Preprocessing/Imputer.php
+++ b/src/Phpml/Preprocessing/Imputer.php
@@ -26,16 +26,23 @@ class Imputer implements Preprocessor
      */
     private $axis;
 
+    /**
+     * @var $samples
+     */
+    private $samples;
+
     /**
      * @param mixed    $missingValue
      * @param Strategy $strategy
      * @param int      $axis
+     * @param array|null $samples
      */
-    public function __construct($missingValue = null, Strategy $strategy, int $axis = self::AXIS_COLUMN)
+    public function __construct($missingValue = null, Strategy $strategy, int $axis = self::AXIS_COLUMN, array $samples = [])
     {
         $this->missingValue = $missingValue;
         $this->strategy = $strategy;
         $this->axis = $axis;
+        $this->samples = $samples;
     }
 
     /**
@@ -43,7 +50,7 @@ class Imputer implements Preprocessor
      */
     public function fit(array $samples)
     {
-        // TODO: Implement fit() method.
+        $this->samples = $samples;
     }
 
     /**
@@ -52,19 +59,18 @@ class Imputer implements Preprocessor
     public function transform(array &$samples)
     {
         foreach ($samples as &$sample) {
-            $this->preprocessSample($sample, $samples);
+            $this->preprocessSample($sample);
         }
     }
 
     /**
      * @param array $sample
-     * @param array $samples
      */
-    private function preprocessSample(array &$sample, array $samples)
+    private function preprocessSample(array &$sample)
     {
         foreach ($sample as $column => &$value) {
             if ($value === $this->missingValue) {
-                $value = $this->strategy->replaceValue($this->getAxis($column, $sample, $samples));
+                $value = $this->strategy->replaceValue($this->getAxis($column, $sample));
             }
         }
     }
@@ -72,18 +78,17 @@ class Imputer implements Preprocessor
     /**
      * @param int   $column
      * @param array $currentSample
-     * @param array $samples
      * 
      * @return array
      */
-    private function getAxis(int $column, array $currentSample, array $samples): array
+    private function getAxis(int $column, array $currentSample): array
     {
         if (self::AXIS_ROW === $this->axis) {
             return array_diff($currentSample, [$this->missingValue]);
         }
 
         $axis = [];
-        foreach ($samples as $sample) {
+        foreach ($this->samples as $sample) {
             if ($sample[$column] !== $this->missingValue) {
                 $axis[] = $sample[$column];
             }
diff --git a/src/Phpml/Preprocessing/Normalizer.php b/src/Phpml/Preprocessing/Normalizer.php
index 11a0218..7647997 100644
--- a/src/Phpml/Preprocessing/Normalizer.php
+++ b/src/Phpml/Preprocessing/Normalizer.php
@@ -5,7 +5,6 @@ declare (strict_types = 1);
 namespace Phpml\Preprocessing;
 
 use Phpml\Exception\NormalizerException;
-use Phpml\Exception\PreprocessorException;
 
 class Normalizer implements Preprocessor
 {
@@ -33,12 +32,10 @@ class Normalizer implements Preprocessor
 
     /**
      * @param array $samples
-     * 
-     * @throws PreprocessorException
      */
     public function fit(array $samples)
     {
-        throw PreprocessorException::fitNotAllowed();
+        // intentionally not implemented
     }
 
     /**
diff --git a/tests/Phpml/Preprocessing/ImputerTest.php b/tests/Phpml/Preprocessing/ImputerTest.php
index a7e36f7..9aa3ea3 100644
--- a/tests/Phpml/Preprocessing/ImputerTest.php
+++ b/tests/Phpml/Preprocessing/ImputerTest.php
@@ -27,7 +27,7 @@ class ImputerTest extends \PHPUnit_Framework_TestCase
             [8, 7, 4, 5],
         ];
 
-        $imputer = new Imputer(null, new MeanStrategy(), Imputer::AXIS_COLUMN);
+        $imputer = new Imputer(null, new MeanStrategy(), Imputer::AXIS_COLUMN, $data);
         $imputer->transform($data);
 
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
@@ -49,7 +49,7 @@ class ImputerTest extends \PHPUnit_Framework_TestCase
             [8, 7, 6.66, 5],
         ];
 
-        $imputer = new Imputer(null, new MeanStrategy(), Imputer::AXIS_ROW);
+        $imputer = new Imputer(null, new MeanStrategy(), Imputer::AXIS_ROW, $data);
         $imputer->transform($data);
 
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
@@ -71,7 +71,7 @@ class ImputerTest extends \PHPUnit_Framework_TestCase
             [8, 7, 3, 5],
         ];
 
-        $imputer = new Imputer(null, new MedianStrategy(), Imputer::AXIS_COLUMN);
+        $imputer = new Imputer(null, new MedianStrategy(), Imputer::AXIS_COLUMN, $data);
         $imputer->transform($data);
 
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
@@ -93,7 +93,7 @@ class ImputerTest extends \PHPUnit_Framework_TestCase
             [8, 7, 7, 5],
         ];
 
-        $imputer = new Imputer(null, new MedianStrategy(), Imputer::AXIS_ROW);
+        $imputer = new Imputer(null, new MedianStrategy(), Imputer::AXIS_ROW, $data);
         $imputer->transform($data);
 
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
@@ -117,7 +117,7 @@ class ImputerTest extends \PHPUnit_Framework_TestCase
             [8, 3, 2, 5],
         ];
 
-        $imputer = new Imputer(null, new MostFrequentStrategy(), Imputer::AXIS_COLUMN);
+        $imputer = new Imputer(null, new MostFrequentStrategy(), Imputer::AXIS_COLUMN, $data);
         $imputer->transform($data);
 
         $this->assertEquals($imputeData, $data);
@@ -141,7 +141,7 @@ class ImputerTest extends \PHPUnit_Framework_TestCase
             [8, 3, 2, 5, 4],
         ];
 
-        $imputer = new Imputer(null, new MostFrequentStrategy(), Imputer::AXIS_ROW);
+        $imputer = new Imputer(null, new MostFrequentStrategy(), Imputer::AXIS_ROW, $data);
         $imputer->transform($data);
 
         $this->assertEquals($imputeData, $data);