diff --git a/src/Phpml/Classification/DecisionTree.php b/src/Phpml/Classification/DecisionTree.php
index b2b4db3..0a70d2f 100644
--- a/src/Phpml/Classification/DecisionTree.php
+++ b/src/Phpml/Classification/DecisionTree.php
@@ -16,11 +16,6 @@ class DecisionTree implements Classifier
     const CONTINUOS = 1;
     const NOMINAL = 2;
 
-    /**
-     * @var array
-     */
-    private $samples = [];
-
     /**
      * @var array
      */
diff --git a/src/Phpml/Classification/Linear/DecisionStump.php b/src/Phpml/Classification/Linear/DecisionStump.php
index 1605a20..de86fe9 100644
--- a/src/Phpml/Classification/Linear/DecisionStump.php
+++ b/src/Phpml/Classification/Linear/DecisionStump.php
@@ -5,13 +5,13 @@ declare(strict_types=1);
 namespace Phpml\Classification\Linear;
 
 use Phpml\Helper\Predictable;
-use Phpml\Helper\Trainable;
+use Phpml\Helper\OneVsRest;
 use Phpml\Classification\WeightedClassifier;
 use Phpml\Classification\DecisionTree;
 
 class DecisionStump extends WeightedClassifier
 {
-    use Trainable, Predictable;
+    use Predictable, OneVsRest;
 
     const AUTO_SELECT = -1;
 
@@ -20,6 +20,10 @@ class DecisionStump extends WeightedClassifier
      */
     protected $givenColumnIndex;
 
+    /**
+     * @var array
+     */
+    protected $binaryLabels;
 
     /**
      * Sample weights : If used the optimization on the decision value
@@ -57,10 +61,22 @@ class DecisionStump extends WeightedClassifier
      */
     protected $columnTypes;
 
+    /**
+     * @var int
+     */
+    protected $featureCount;
+
     /**
      * @var float
      */
-    protected $numSplitCount = 10.0;
+    protected $numSplitCount = 100.0;
+
+    /**
+     * Distribution of samples in the leaves
+     *
+     * @var array
+     */
+    protected $prob;
 
     /**
      * A DecisionStump classifier is a one-level deep DecisionTree. It is generally
@@ -81,20 +97,15 @@ class DecisionStump extends WeightedClassifier
      * @param array $samples
      * @param array $targets
      */
-    public function train(array $samples, array $targets)
+    protected function trainBinary(array $samples, array $targets)
     {
         $this->samples = array_merge($this->samples, $samples);
         $this->targets = array_merge($this->targets, $targets);
-
-        // DecisionStump is capable of classifying between two classes only
-        $labels = array_count_values($this->targets);
-        $this->labels = array_keys($labels);
-        if (count($this->labels) != 2) {
-            throw new \Exception("DecisionStump can classify between two classes only:" . implode(',', $this->labels));
-        }
+        $this->binaryLabels = array_keys(array_count_values($this->targets));
+        $this->featureCount = count($this->samples[0]);
 
         // If a column index is given, it should be among the existing columns
-        if ($this->givenColumnIndex > count($samples[0]) - 1) {
+        if ($this->givenColumnIndex > count($this->samples[0]) - 1) {
             $this->givenColumnIndex = self::AUTO_SELECT;
         }
 
@@ -106,7 +117,7 @@ class DecisionStump extends WeightedClassifier
                 throw new \Exception("Number of sample weights does not match with number of samples");
             }
         } else {
-            $this->weights = array_fill(0, count($samples), 1);
+            $this->weights = array_fill(0, count($this->samples), 1);
         }
 
         // Determine type of each column as either "continuous" or "nominal"
@@ -114,14 +125,15 @@ class DecisionStump extends WeightedClassifier
 
         // Try to find the best split in the columns of the dataset
         // by calculating error rate for each split point in each column
-        $columns = range(0, count($samples[0]) - 1);
+        $columns = range(0, count($this->samples[0]) - 1);
         if ($this->givenColumnIndex != self::AUTO_SELECT) {
             $columns = [$this->givenColumnIndex];
         }
 
         $bestSplit = [
             'value' => 0, 'operator' => '',
-            'column' => 0, 'trainingErrorRate' => 1.0];
+            'prob' => [], 'column' => 0,
+            'trainingErrorRate' => 1.0];
         foreach ($columns as $col) {
             if ($this->columnTypes[$col] == DecisionTree::CONTINUOS) {
                 $split = $this->getBestNumericalSplit($col);
@@ -164,6 +176,10 @@ class DecisionStump extends WeightedClassifier
     protected function getBestNumericalSplit(int $col)
     {
         $values = array_column($this->samples, $col);
+        // Trying all possible points may be accomplished in two general ways:
+        // 1- Try all values in the $samples array ($values)
+        // 2- Artificially split the range of values into several parts and try them
+        // We choose the second one because it is faster in larger datasets
         $minValue = min($values);
         $maxValue = max($values);
         $stepSize = ($maxValue - $minValue) / $this->numSplitCount;
@@ -174,19 +190,21 @@ class DecisionStump extends WeightedClassifier
             // Before trying all possible split points, let's first try
             // the average value for the cut point
             $threshold = array_sum($values) / (float) count($values);
-            $errorRate = $this->calculateErrorRate($threshold, $operator, $values);
+            list($errorRate, $prob) = $this->calculateErrorRate($threshold, $operator, $values);
             if ($split == null || $errorRate < $split['trainingErrorRate']) {
                 $split = ['value' => $threshold, 'operator' => $operator,
-                        'column' => $col, 'trainingErrorRate' => $errorRate];
+                        'prob' => $prob, 'column' => $col,
+                        'trainingErrorRate' => $errorRate];
             }
 
             // Try other possible points one by one
             for ($step = $minValue; $step <= $maxValue; $step+= $stepSize) {
                 $threshold = (float)$step;
-                $errorRate = $this->calculateErrorRate($threshold, $operator, $values);
+                list($errorRate, $prob) = $this->calculateErrorRate($threshold, $operator, $values);
                 if ($errorRate < $split['trainingErrorRate']) {
                     $split = ['value' => $threshold, 'operator' => $operator,
-                        'column' => $col, 'trainingErrorRate' => $errorRate];
+                        'prob' => $prob, 'column' => $col,
+                        'trainingErrorRate' => $errorRate];
                 }
             }// for
         }
@@ -210,11 +228,12 @@ class DecisionStump extends WeightedClassifier
 
         foreach (['=', '!='] as $operator) {
             foreach ($distinctVals as $val) {
-                $errorRate = $this->calculateErrorRate($val, $operator, $values);
+                list($errorRate, $prob) = $this->calculateErrorRate($val, $operator, $values);
 
                 if ($split == null || $split['trainingErrorRate'] < $errorRate) {
                     $split = ['value' => $val, 'operator' => $operator,
-                        'column' => $col, 'trainingErrorRate' => $errorRate];
+                        'prob' => $prob, 'column' => $col,
+                        'trainingErrorRate' => $errorRate];
                 }
             }// for
         }
@@ -238,9 +257,9 @@ class DecisionStump extends WeightedClassifier
             case '>=': return $leftValue >= $rightValue;
             case '<': return $leftValue < $rightValue;
             case '<=': return $leftValue <= $rightValue;
-            case '=': return $leftValue == $rightValue;
+            case '=': return $leftValue === $rightValue;
             case '!=':
-            case '<>': return $leftValue != $rightValue;
+            case '<>': return $leftValue !== $rightValue;
         }
 
         return false;
@@ -253,42 +272,90 @@ class DecisionStump extends WeightedClassifier
      * @param float $threshold
      * @param string $operator
      * @param array $values
+     *
+     * @return array
      */
     protected function calculateErrorRate(float $threshold, string $operator, array $values)
     {
-        $total = (float) array_sum($this->weights);
         $wrong = 0.0;
-        $leftLabel = $this->labels[0];
-        $rightLabel= $this->labels[1];
+        $prob = [];
+        $leftLabel = $this->binaryLabels[0];
+        $rightLabel= $this->binaryLabels[1];
+
         foreach ($values as $index => $value) {
-            if ($this->evaluate($threshold, $operator, $value)) {
+            if ($this->evaluate($value, $operator, $threshold)) {
                 $predicted = $leftLabel;
             } else {
                 $predicted = $rightLabel;
             }
 
-            if ($predicted != $this->targets[$index]) {
+            $target = $this->targets[$index];
+            if (strval($predicted) != strval($this->targets[$index])) {
                 $wrong += $this->weights[$index];
             }
+
+            if (! isset($prob[$predicted][$target])) {
+                $prob[$predicted][$target] = 0;
+            }
+            $prob[$predicted][$target]++;
+        }
+
+        // Calculate probabilities: Proportion of labels in each leaf
+        $dist = array_combine($this->binaryLabels, array_fill(0, 2, 0.0));
+        foreach ($prob as $leaf => $counts) {
+            $leafTotal = (float)array_sum($prob[$leaf]);
+            foreach ($counts as $label => $count) {
+                if (strval($leaf) == strval($label)) {
+                    $dist[$leaf] = $count / $leafTotal;
+                }
+            }
         }
 
-        return $wrong / $total;
+        return [$wrong / (float) array_sum($this->weights), $dist];
+    }
+
+    /**
+     * Returns the probability of the sample of belonging to the given label
+     *
+     * Probability of a sample is calculated as the proportion of the label
+     * within the labels of the training samples in the decision node
+     *
+     * @param array $sample
+     * @param mixed $label
+     *
+     * @return float
+     */
+    protected function predictProbability(array $sample, $label)
+    {
+        $predicted = $this->predictSampleBinary($sample);
+        if (strval($predicted) == strval($label)) {
+            return $this->prob[$label];
+        }
+
+        return 0.0;
     }
 
     /**
      * @param array $sample
+     *
      * @return mixed
      */
-    protected function predictSample(array $sample)
+    protected function predictSampleBinary(array $sample)
     {
-        if ($this->evaluate($this->value, $this->operator, $sample[$this->column])) {
-            return $this->labels[0];
+        if ($this->evaluate($sample[$this->column], $this->operator, $this->value)) {
+            return $this->binaryLabels[0];
         }
-        return $this->labels[1];
+
+        return $this->binaryLabels[1];
     }
 
+    /**
+     * @return string
+     */
     public function __toString()
     {
-        return "$this->column $this->operator $this->value";
+        return "IF $this->column $this->operator $this->value " .
+            "THEN " . $this->binaryLabels[0] . " ".
+            "ELSE " . $this->binaryLabels[1];
     }
 }
diff --git a/src/Phpml/Classification/Linear/Perceptron.php b/src/Phpml/Classification/Linear/Perceptron.php
index e2c684d..32e41f2 100644
--- a/src/Phpml/Classification/Linear/Perceptron.php
+++ b/src/Phpml/Classification/Linear/Perceptron.php
@@ -5,12 +5,13 @@ declare(strict_types=1);
 namespace Phpml\Classification\Linear;
 
 use Phpml\Helper\Predictable;
+use Phpml\Helper\OneVsRest;
 use Phpml\Classification\Classifier;
 use Phpml\Preprocessing\Normalizer;
 
 class Perceptron implements Classifier
 {
-    use Predictable;
+    use Predictable, OneVsRest;
 
     /**
      * The function whose result will be used to calculate the network error
@@ -114,7 +115,7 @@ class Perceptron implements Classifier
      * @param array $samples
      * @param array $targets
      */
-    public function train(array $samples, array $targets)
+    public function trainBinary(array $samples, array $targets)
     {
         $this->labels = array_keys(array_count_values($targets));
         if (count($this->labels) > 2) {
@@ -128,7 +129,7 @@ class Perceptron implements Classifier
         // Set all target values to either -1 or 1
         $this->labels = [1 => $this->labels[0], -1 => $this->labels[1]];
         foreach ($targets as $target) {
-            $this->targets[] = $target == $this->labels[1] ? 1 : -1;
+            $this->targets[] = strval($target) == strval($this->labels[1]) ? 1 : -1;
         }
 
         // Set samples and feature count vars
@@ -213,6 +214,25 @@ class Perceptron implements Classifier
         return false;
     }
 
+    /**
+     * Checks if the sample should be normalized and if so, returns the
+     * normalized sample
+     *
+     * @param array $sample
+     *
+     * @return array
+     */
+    protected function checkNormalizedSample(array $sample)
+    {
+        if ($this->normalizer) {
+            $samples = [$sample];
+            $this->normalizer->transform($samples);
+            $sample = $samples[0];
+        }
+
+        return $sample;
+    }
+
     /**
      * Calculates net output of the network as a float value for the given input
      *
@@ -244,17 +264,34 @@ class Perceptron implements Classifier
         return $this->output($sample) > 0 ? 1 : -1;
     }
 
+    /**
+     * Returns the probability of the sample of belonging to the given label.
+     *
+     * The probability is simply taken as the distance of the sample
+     * to the decision plane.
+     *
+     * @param array $sample
+     * @param mixed $label
+     */
+    protected function predictProbability(array $sample, $label)
+    {
+        $predicted = $this->predictSampleBinary($sample);
+
+        if (strval($predicted) == strval($label)) {
+            $sample = $this->checkNormalizedSample($sample);
+            return abs($this->output($sample));
+        }
+
+        return 0.0;
+    }
+
     /**
      * @param array $sample
      * @return mixed
      */
-    protected function predictSample(array $sample)
+    protected function predictSampleBinary(array $sample)
     {
-        if ($this->normalizer) {
-            $samples = [$sample];
-            $this->normalizer->transform($samples);
-            $sample = $samples[0];
-        }
+        $sample = $this->checkNormalizedSample($sample);
 
         $predictedClass = $this->outputClass($sample);
 
diff --git a/src/Phpml/Helper/OneVsRest.php b/src/Phpml/Helper/OneVsRest.php
new file mode 100644
index 0000000..1288363
--- /dev/null
+++ b/src/Phpml/Helper/OneVsRest.php
@@ -0,0 +1,126 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\Helper;
+
+trait OneVsRest
+{
+    /**
+     * @var array
+     */
+    protected $samples = [];
+
+    /**
+     * @var array
+     */
+    protected $targets = [];
+    
+    /**
+     * @var array
+     */
+    protected $classifiers;
+
+    /**
+     * @var array
+     */
+    protected $labels;
+
+    /**
+     * Train a binary classifier in the OvR style
+     *
+     * @param array $samples
+     * @param array $targets
+     */
+    public function train(array $samples, array $targets)
+    {
+        // Clone the current classifier, so that
+        // we don't mess up its variables while training
+        // multiple instances of this classifier
+        $classifier = clone $this;
+        $this->classifiers = [];
+
+        // If there are only two targets, then there is no need to perform OvR
+        $this->labels = array_keys(array_count_values($targets));
+        if (count($this->labels) == 2) {
+            $classifier->trainBinary($samples, $targets);
+            $this->classifiers[] = $classifier;
+        } else {
+            // Train a separate classifier for each label and memorize them
+            $this->samples = $samples;
+            $this->targets = $targets;
+            foreach ($this->labels as $label) {
+                $predictor = clone $classifier;
+                $targets = $this->binarizeTargets($label);
+                $predictor->trainBinary($samples, $targets);
+                $this->classifiers[$label] = $predictor;
+            }
+        }
+    }
+
+    /**
+     * Groups all targets into two groups: Targets equal to
+     * the given label and the others
+     *
+     * @param mixed $label
+     */
+    private function binarizeTargets($label)
+    {
+        $targets = [];
+
+        foreach ($this->targets as $target) {
+            $targets[] = $target == $label ? $label : "not_$label";
+        }
+
+        return $targets;
+    }
+
+
+    /**
+     * @param array $sample
+     *
+     * @return mixed
+     */
+    protected function predictSample(array $sample)
+    {
+        if (count($this->labels) == 2) {
+            return $this->classifiers[0]->predictSampleBinary($sample);
+        }
+
+        $probs = [];
+
+        foreach ($this->classifiers as $label => $predictor) {
+            $probs[$label] = $predictor->predictProbability($sample, $label);
+        }
+
+        arsort($probs, SORT_NUMERIC);
+        return key($probs);
+    }
+
+    /**
+     * Each classifier should implement this method instead of train(samples, targets)
+     *
+     * @param array $samples
+     * @param array $targets
+     */
+    abstract protected function trainBinary(array $samples, array $targets);
+
+    /**
+     * Each classifier that make use of OvR approach should be able to
+     * return a probability for a sample to belong to the given label.
+     *
+     * @param array $sample
+     *
+     * @return mixed
+     */
+    abstract protected function predictProbability(array $sample, string $label);
+
+    /**
+     * Each classifier should implement this method instead of predictSample()
+     *
+     * @param array $sample
+     *
+     * @return mixed
+     */
+    abstract protected function predictSampleBinary(array $sample);
+}
diff --git a/src/Phpml/Math/Statistic/Gaussian.php b/src/Phpml/Math/Statistic/Gaussian.php
new file mode 100644
index 0000000..df27f07
--- /dev/null
+++ b/src/Phpml/Math/Statistic/Gaussian.php
@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\Math\Statistic;
+
+class Gaussian
+{
+    /**
+     * @var float
+     */
+    protected $mean;
+
+    /**
+     * @var float
+     */
+    protected $std;
+
+    /**
+     * @param float $mean
+     * @param float $std
+     */
+    public function __construct(float $mean, float $std)
+    {
+        $this->mean = $mean;
+        $this->std = $std;
+    }
+
+    /**
+     * Returns probability density of the given <i>$value</i>
+     *
+     * @param float $value
+     *
+     * @return type
+     */
+    public function pdf(float $value)
+    {
+        // Calculate the probability density by use of normal/Gaussian distribution
+        // Ref: https://en.wikipedia.org/wiki/Normal_distribution
+        $std2 = $this->std ** 2;
+        $mean = $this->mean;
+        return exp(- (($value - $mean) ** 2) / (2 * $std2)) / sqrt(2 * $std2 * pi());
+    }
+
+    /**
+     * Returns probability density value of the given <i>$value</i> based on
+     * given standard deviation and the mean
+     *
+     * @param float $mean
+     * @param float $std
+     * @param float $value
+     *
+     * @return float
+     */
+    public static function distributionPdf(float $mean, float $std, float $value)
+    {
+        $normal = new self($mean, $std);
+        return $normal->pdf($value);
+    }
+}
diff --git a/tests/Phpml/Classification/Linear/AdalineTest.php b/tests/Phpml/Classification/Linear/AdalineTest.php
index 7ea63ab..c07fbd2 100644
--- a/tests/Phpml/Classification/Linear/AdalineTest.php
+++ b/tests/Phpml/Classification/Linear/AdalineTest.php
@@ -30,6 +30,21 @@ class AdalineTest extends TestCase
         $this->assertEquals(1, $classifier->predict([0.1, 0.99]));
         $this->assertEquals(1, $classifier->predict([1.1, 0.8]));
 
+        // By use of One-v-Rest, Adaline can perform multi-class classification
+        // The samples should be separable by lines perpendicular to the dimensions
+        $samples = [
+            [0, 0], [0, 1], [1, 0], [1, 1], // First group : a cluster at bottom-left corner in 2D
+            [5, 5], [6, 5], [5, 6], [7, 5], // Second group: another cluster at the middle-right
+            [3, 10],[3, 10],[3, 8], [3, 9]  // Third group : cluster at the top-middle
+        ];
+        $targets = [0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2];
+
+        $classifier = new Adaline();
+        $classifier->train($samples, $targets);
+        $this->assertEquals(0, $classifier->predict([0.5, 0.5]));
+        $this->assertEquals(1, $classifier->predict([6.0, 5.0]));
+        $this->assertEquals(2, $classifier->predict([3.0, 9.5]));
+
         return $classifier;
     }
 
diff --git a/tests/Phpml/Classification/Linear/DecisionStumpTest.php b/tests/Phpml/Classification/Linear/DecisionStumpTest.php
index f83e095..4060ce3 100644
--- a/tests/Phpml/Classification/Linear/DecisionStumpTest.php
+++ b/tests/Phpml/Classification/Linear/DecisionStumpTest.php
@@ -12,8 +12,9 @@ class DecisionStumpTest extends TestCase
 {
     public function testPredictSingleSample()
     {
-        // Samples should be separable with a line perpendicular to any dimension
-        // given in the dataset
+        // Samples should be separable with a line perpendicular
+        // to any dimension given in the dataset
+        //
         // First: horizontal test
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1]];
         $targets = [0, 0, 1, 1];
@@ -34,6 +35,21 @@ class DecisionStumpTest extends TestCase
         $this->assertEquals(1, $classifier->predict([1.0, 0.99]));
         $this->assertEquals(1, $classifier->predict([1.1, 0.1]));
 
+        // By use of One-v-Rest, DecisionStump can perform multi-class classification
+        // The samples should be separable by lines perpendicular to the dimensions
+        $samples = [
+            [0, 0], [0, 1], [1, 0], [1, 1], // First group : a cluster at bottom-left corner in 2D
+            [5, 5], [6, 5], [5, 6], [7, 5], // Second group: another cluster at the middle-right
+            [3, 10],[3, 10],[3, 8], [3, 9]  // Third group : cluster at the top-middle
+        ];
+        $targets = [0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2];
+
+        $classifier = new DecisionStump();
+        $classifier->train($samples, $targets);
+        $this->assertEquals(0, $classifier->predict([0.5, 0.5]));
+        $this->assertEquals(1, $classifier->predict([6.0, 5.0]));
+        $this->assertEquals(2, $classifier->predict([3.5, 9.5]));
+
         return $classifier;
     }
 
diff --git a/tests/Phpml/Classification/Linear/PerceptronTest.php b/tests/Phpml/Classification/Linear/PerceptronTest.php
index 64954f7..ef820fc 100644
--- a/tests/Phpml/Classification/Linear/PerceptronTest.php
+++ b/tests/Phpml/Classification/Linear/PerceptronTest.php
@@ -30,6 +30,21 @@ class PerceptronTest extends TestCase
         $this->assertEquals(1, $classifier->predict([0.1, 0.99]));
         $this->assertEquals(1, $classifier->predict([1.1, 0.8]));
 
+        // By use of One-v-Rest, Perceptron can perform multi-class classification
+        // The samples should be separable by lines perpendicular to the dimensions
+        $samples = [
+            [0, 0], [0, 1], [1, 0], [1, 1], // First group : a cluster at bottom-left corner in 2D
+            [5, 5], [6, 5], [5, 6], [7, 5], // Second group: another cluster at the middle-right
+            [3, 10],[3, 10],[3, 8], [3, 9]  // Third group : cluster at the top-middle
+        ];
+        $targets = [0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2];
+
+        $classifier = new Perceptron();
+        $classifier->train($samples, $targets);
+        $this->assertEquals(0, $classifier->predict([0.5, 0.5]));
+        $this->assertEquals(1, $classifier->predict([6.0, 5.0]));
+        $this->assertEquals(2, $classifier->predict([3.0, 9.5]));
+        
         return $classifier;
     }
 
diff --git a/tests/Phpml/Math/Statistic/GaussianTest.php b/tests/Phpml/Math/Statistic/GaussianTest.php
new file mode 100644
index 0000000..6bbf63b
--- /dev/null
+++ b/tests/Phpml/Math/Statistic/GaussianTest.php
@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace test\Phpml\Math\StandardDeviation;
+
+use Phpml\Math\Statistic\Gaussian;
+use PHPUnit\Framework\TestCase;
+
+class GaussianTest extends TestCase
+{
+    public function testPdf()
+    {
+        $std = 1.0;
+        $mean= 0.0;
+        $g = new Gaussian($mean, $std);
+
+        // Allowable error
+        $delta = 0.001;
+        $x     = [0, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0];
+        $pdf = [0.3989, 0.3969, 0.3520, 0.2419, 0.1295, 0.0539, 0.0175, 0.0044];
+        foreach ($x as $i => $v) {
+            $this->assertEquals($pdf[$i], $g->pdf($v), '', $delta);
+
+            $this->assertEquals($pdf[$i], Gaussian::distributionPdf($mean, $std, $v), '', $delta);
+        }
+    }
+}