diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 000000000..4bcedf401
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,33 @@
+* text=false
+
+# Force the following filetypes to have unix eols, so Windows does not break them
+*.php text eol=lf
+*.js text eol=lf
+*.sh text eol=lf
+*.css text eol=lf
+*.md text eol=lf
+*.txt text eol=lf
+*.htaccess text eol=lf
+*.json text eol=lf
+
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.gif binary
+*.ico binary
+*.mov binary
+*.mp4 binary
+*.mp3 binary
+*.flv binary
+*.fla binary
+*.swf binary
+*.gz binary
+*.zip binary
+*.7z binary
+*.ttf binary
+*.eot binary
+*.woff binary
+*.pyc binary
+*.pdf binary
+*.dat binary
+*.z binary
\ No newline at end of file
diff --git a/Math/Functions/Functions.php b/Math/Functions/Functions.php
index 82e96cf0f..79ca16a8a 100644
--- a/Math/Functions/Functions.php
+++ b/Math/Functions/Functions.php
@@ -15,7 +15,7 @@ declare(strict_types=1);
 namespace phpOMS\Math\Functions;
 
 /**
- * Well known functions class.
+ * Well known functions and helpers class.
  *
  * @package    Framework
  * @license    OMS License 1.0
@@ -241,6 +241,50 @@ class Functions
     {
         return !((bool) ($a & 1));
     }
+    
+    /**
+     * Power all values in array.
+     *
+     * @param array $values Values to square
+     * @param float $exp    Exponent
+     *
+     * @return array
+     *
+     * @since  1.0.0
+     *         todo: move to utils?! implement sqrt for array as well... could be usefull for others (e.g. matrix)
+     */
+    public static function powerFloat(array $values, float $exp = 2.0) : array
+    {
+        $squared = [];
+
+        foreach ($values as $value) {
+            $squared[] = $value ** $exp;
+        }
+
+        return $squared;
+    }
+    
+    /**
+     * Power all values in array.
+     *
+     * @param array $values Values to square
+     * @param int   $exp    Exponent
+     *
+     * @return array
+     *
+     * @since  1.0.0
+     *         todo: move to utils?! implement sqrt for array as well... could be usefull for others (e.g. matrix)
+     */
+    public static function powerInt(array $values, int $exp = 2) : array
+    {
+        $squared = [];
+
+        foreach ($values as $value) {
+            $squared[] = $value ** $exp;
+        }
+
+        return $squared;
+    }
 
     /**
      * Gets the relative position on a circular construct.
diff --git a/Math/Statistic/Forecast/Error.php b/Math/Statistic/Forecast/Error.php
index 238cafb28..e8c0ff10b 100644
--- a/Math/Statistic/Forecast/Error.php
+++ b/Math/Statistic/Forecast/Error.php
@@ -111,7 +111,7 @@ class Error
      */
     public static function getMeanAbsoulteError(array $errors) : float
     {
-        return Average::arithmeticMean(Functions::abs($errors));
+        return MeasureOfDispersion::meanAbsoluteDeviation($errors);
     }
 
     /**
@@ -125,7 +125,7 @@ class Error
      */
     public static function getMeanSquaredError(array $errors) : float
     {
-        return Average::arithmeticMean(self::square($errors));
+        return MeasureOfDispersion::squaredMeanDeviation($errors);
     }
 
     /**
@@ -139,7 +139,7 @@ class Error
      */
     public static function getRootMeanSquaredError(array $errors) : float
     {
-        return sqrt(Average::arithmeticMean(self::square($errors)));
+        return sqrt(Average::arithmeticMean(Functions::powerInt($errors, 2)));
     }
 
     /**
@@ -297,27 +297,6 @@ class Error
         return Average::arithmeticMean($error);
     }
 
-    /**
-     * Square all values in array.
-     *
-     * @param array $values Values to square
-     *
-     * @return array
-     *
-     * @since  1.0.0
-     *         todo: move to utils?! implement sqrt for array as well... could be usefull for others (e.g. matrix)
-     */
-    private static function square(array $values) : array
-    {
-        $squared = [];
-
-        foreach ($values as $value) {
-            $squared[] = $value * $value;
-        }
-
-        return $squared;
-    }
-
     /**
      * Get cross sectional scaled errors (CSSE)
      *
@@ -387,7 +366,7 @@ class Error
      */
     public static function getMeanSquaredScaledError(array $scaledErrors) : float
     {
-        return Average::arithmeticMean(self::square($scaledErrors));
+        return Average::arithmeticMean(Functions::powerInt($scaledErrors, 2));
     }
 
     /**
diff --git a/Math/Statistic/MeasureOfDispersion.php b/Math/Statistic/MeasureOfDispersion.php
index 4481a6b1d..5eb1d801a 100644
--- a/Math/Statistic/MeasureOfDispersion.php
+++ b/Math/Statistic/MeasureOfDispersion.php
@@ -54,6 +54,7 @@ class MeasureOfDispersion
      * Example: ([4, 5, 9, 1, 3])
      *
      * @param array $values Values
+     * @param float $mean   Mean
      *
      * @return float
      *
@@ -61,9 +62,9 @@ class MeasureOfDispersion
      *
      * @since  1.0.0
      */
-    public static function empiricalVariationCoefficient(array $values) : float
+    public static function empiricalVariationCoefficient(array $values, float $mean = null) : float
     {
-        $mean = Average::arithmeticMean($values);
+        $mean = isset($mean) ? $mean : Average::arithmeticMean($values);
 
         if ($mean === 0) {
             throw new ZeroDevisionException();
@@ -80,14 +81,15 @@ class MeasureOfDispersion
      * @latex \sigma = \sqrt{\sigma^{2}} = \sqrt{Var(X)}
      *
      * @param array $values Values
+     * @param float $mean   Mean
      *
      * @return float
      *
      * @since  1.0.0
      */
-    public static function standardDeviation(array $values) : float
+    public static function standardDeviation(array $values, float $mean = null) : float
     {
-        $mean = Average::arithmeticMean($values);
+        $mean = isset($mean) ? $mean : Average::arithmeticMean($values);
         $sum  = 0.0;
 
         foreach ($values as $value) {
@@ -107,6 +109,7 @@ class MeasureOfDispersion
      * @latex \sigma^{2} = Var(X) = \frac{1}{N - 1} \sum_{i = 1}^{N}\left(x_{i} - \bar{X}\right)^{2}
      *
      * @param array $values Values
+     * @param float $mean   Mean
      *
      * @return float
      *
@@ -114,7 +117,7 @@ class MeasureOfDispersion
      *
      * @since  1.0.0
      */
-    public static function sampleVariance(array $values) : float
+    public static function sampleVariance(array $values, float $mean = null) : float
     {
         $count = count($values);
 
@@ -122,7 +125,7 @@ class MeasureOfDispersion
             throw new ZeroDevisionException();
         }
 
-        return self::empiricalVariance($values) * $count / ($count - 1);
+        return self::empiricalVariance($values, [], $mean) * $count / ($count - 1);
     }
 
     /**
@@ -136,6 +139,7 @@ class MeasureOfDispersion
      *
      * @param array $values        Values
      * @param array $probabilities Probabilities
+     * @param float $mean          Mean
      *
      * @return float
      *
@@ -143,7 +147,7 @@ class MeasureOfDispersion
      *
      * @since  1.0.0
      */
-    public static function empiricalVariance(array $values, array $probabilities = []) : float
+    public static function empiricalVariance(array $values, array $probabilities = [], float $mean = null) : float
     {
         $count          = count($values);
         $hasProbability = !empty($probabilities);
@@ -152,7 +156,7 @@ class MeasureOfDispersion
             throw new ZeroDevisionException();
         }
 
-        $mean = $hasProbability ? Average::weightedAverage($values, $probabilities) : Average::arithmeticMean($values);
+        $mean = $hasProbability ? Average::weightedAverage($values, $probabilities) : (isset($mean) ? $mean : Average::arithmeticMean($values));
         $sum  = 0;
 
         foreach ($values as $key => $value) {
@@ -169,8 +173,10 @@ class MeasureOfDispersion
      * 
      * @latex \sigma_{XY} = cov(X, Y) = \sum_{i = 1}^{N}\frac{\left(x_{i} - \bar{X}\right) \left(y_{i} - \bar{Y}\right)}{N - 1}
      *
-     * @param array $x Values
-     * @param array $y Values
+     * @param array $x     Values
+     * @param array $y     Values
+     * @param array $meanX Mean
+     * @param array $meanY Mean
      *
      * @return float
      *
@@ -178,7 +184,7 @@ class MeasureOfDispersion
      *
      * @since  1.0.0
      */
-    public static function empiricalCovariance(array $x, array $y) : float
+    public static function empiricalCovariance(array $x, array $y, float $meanX = null, float $meanY = null) : float
     {
         $count = count($x);
 
@@ -190,8 +196,8 @@ class MeasureOfDispersion
             throw new InvalidDimensionException($count . 'x' . count($y));
         }
 
-        $xMean = Average::arithmeticMean($x);
-        $yMean = Average::arithmeticMean($y);
+        $xMean = isset($meanX) ? $meanX : Average::arithmeticMean($x);
+        $yMean = isset($meanY) ? $meanY : Average::arithmeticMean($y);
 
         $sum = 0.0;
 
@@ -219,15 +225,16 @@ class MeasureOfDispersion
     /**
      * Get mean deviation.
      *
-     * @param array $x Values
+     * @param array $x    Values
+     * @param float $mean Mean
      *
      * @return float
      *
      * @since  1.0.0
      */
-    public static function meanDeviation(array $x) : float
+    public static function meanDeviation(array $x, float $mean = null) : float
     {
-        $mean = Average::arithmeticMean($x);
+        $mean = isset($mean) ? $mean : Average::arithmeticMean($x);
         $sum  = 0.0;
 
         foreach ($x as $xi) {
@@ -240,15 +247,16 @@ class MeasureOfDispersion
     /**
      * Get mean absolute deviation.
      *
-     * @param array $x Values
+     * @param array $x    Values
+     * @param float $mean Mean
      *
      * @return float
      *
      * @since  1.0.0
      */
-    public static function meanAbsoluteDeviation(array $x) : float
+    public static function meanAbsoluteDeviation(array $x, float $mean = null) : float
     {
-        $mean = Average::arithmeticMean($x);
+        $mean = isset($mean) ? $mean : Average::arithmeticMean($x);
         $sum  = 0.0;
 
         foreach ($x as $xi) {
@@ -261,15 +269,16 @@ class MeasureOfDispersion
     /**
      * Get squared mean deviation.
      *
-     * @param array $x Values
+     * @param array $x    Values
+     * @param float $mean Mean
      *
      * @return float
      *
      * @since  1.0.0
      */
-    public static function squaredMeanDeviation(array $x) : float
+    public static function squaredMeanDeviation(array $x, float $mean = null) : float
     {
-        $mean = Average::arithmeticMean($x);
+        $mean = isset($mean) ? $mean : Average::arithmeticMean($x);
         $sum  = 0.0;
 
         foreach ($x as $xi) {
diff --git a/tests/Math/Functions/FunctionsTest.php b/tests/Math/Functions/FunctionsTest.php
index f88839b89..9fa493b10 100644
--- a/tests/Math/Functions/FunctionsTest.php
+++ b/tests/Math/Functions/FunctionsTest.php
@@ -68,4 +68,13 @@ class FunctionsTest extends \PHPUnit\Framework\TestCase
         self::assertEquals(5, Functions::getRelativeDegree(12, 12, 7));
         self::assertEquals(11, Functions::getRelativeDegree(6, 12, 7));
     }
+    
+    public function testPower()
+    {
+        self::assertEquals([4, 9, 16], Functions::powerInt([2, 3, 4], 2));
+        self::assertEquals([8, 27, 64], Functions::powerInt([2, 3, 4], 3));
+        
+        self::assertEquals([2.0, 3.0, 4.0], Functions::powerFloat([4, 9, 16], 1/2), '', 0.0);
+        self::assertEquals([2.0, 3.0, 4.0], Functions::powerFloat([8, 27, 64], 1/3), '', 0.0);
+    }
 }
diff --git a/tests/Math/Statistic/Forecast/ErrorTest.php b/tests/Math/Statistic/Forecast/ErrorTest.php
index fb3d36c55..cb08f19a9 100644
--- a/tests/Math/Statistic/Forecast/ErrorTest.php
+++ b/tests/Math/Statistic/Forecast/ErrorTest.php
@@ -17,8 +17,54 @@ use phpOMS\Math\Statistic\Forecast\Error;
 
 class ErrorTest extends \PHPUnit\Framework\TestCase
 {
-    public function testPlaceholder()
+    public function testForecastError()
     {
-        self::markTestIncomplete();
+        self::assertEquals(1000 - 700, Error::getForecastError(1000, 700));
+        self::assertEquals(
+            [
+                400 - 300,
+                600 - 700,
+                200 - 200,
+                500 - -300
+            ],
+            Error::getForecastErrorArray(
+                [400, 600, 200, 500],
+                [300, 700, 200, -300]
+            )
+        );
+    }
+    
+    public function testErrorPercentage()
+    {
+        self::assertEquals(300 / 1000, Error::getPercentageError(300, 1000), '', 0.01);
+        self::assertEquals(
+            [
+                (400 - 300) / 400,
+                (600 - 700) / 600,
+                (200 - 200) / 200,
+                (500 - -300) / 500
+            ],
+            Error::getPercentageErrorArray(
+                Error::getForecastErrorArray(
+                    [400, 600, 200, 500],
+                    [300, 700, 200, -300]
+                ),
+                [400, 600, 200, 500]
+            )
+        );
+    }
+    
+    public function testMeanError()
+    {
+        $errors = [
+            400 - 300,
+            600 - 700,
+            200 - 200,
+            500 - -300
+        ];
+        
+        self::assertEquals(300, Error::getMeanAbsoulteError($errors), '', 0.01);
+        self::assertEquals(125000, Error::getMeanSquaredError($errors), '', 0.01);
+        self::assertEquals(406.2019, Error::getRootMeanSquaredError($errors), '', 0.01);
     }
 }