diff --git a/PHPSkills/FactorGraphs/Factor.php b/PHPSkills/FactorGraphs/Factor.php
index 384c43e..ad571e3 100644
--- a/PHPSkills/FactorGraphs/Factor.php
+++ b/PHPSkills/FactorGraphs/Factor.php
@@ -49,7 +49,9 @@ abstract class Factor
     public function updateMessageIndex($messageIndex)
     {
         Guard::argumentIsValidIndex($messageIndex, count($this->_messages), "messageIndex");
-        return $this->updateMessageVariable($this->_messages[$messageIndex], $this->_messageToVariableBinding->getValue($messageIndex));
+        $message = $this->_messages[$messageIndex];
+        $variable = $this->_messageToVariableBinding->getValue($this->_messages[$messageIndex]);
+        return $this->updateMessageVariable($message, $variable);
     }
 
     protected function updateMessageVariable(Message $message, Variable $variable)
@@ -60,7 +62,7 @@ abstract class Factor
     /// Resets the marginal of the variables a factor is connected to
     public function resetMarginals()
     {
-        foreach ($this->_messageToVariableBindings->getAllValues() as $currentVariable)
+        foreach ($this->_messageToVariableBinding->getAllValues() as $currentVariable)
         {
             $currentVariable->resetToPrior();
         }
@@ -69,7 +71,7 @@ abstract class Factor
     /// Sends the ith message to the marginal and returns the log-normalization constant
     public function sendMessageIndex($messageIndex)
     {
-        Guard::argumentIsValidIndex($messageIndex, count($_messages), "messageIndex");
+        Guard::argumentIsValidIndex($messageIndex, count($this->_messages), "messageIndex");
 
         $message = $this->_messages[$messageIndex];
         $variable = $this->_messageToVariableBinding->getValue($message);
diff --git a/PHPSkills/FactorGraphs/FactorGraphLayer.php b/PHPSkills/FactorGraphs/FactorGraphLayer.php
index 0a80ef7..ec0e93a 100644
--- a/PHPSkills/FactorGraphs/FactorGraphLayer.php
+++ b/PHPSkills/FactorGraphs/FactorGraphLayer.php
@@ -43,7 +43,7 @@ abstract class FactorGraphLayer
         $this->_inputVariablesGroups = $value;
     }
 
-    protected function scheduleSequence(&$itemsToSequence, $name)
+    protected function scheduleSequence($itemsToSequence, $name)
     {
         return new ScheduleSequence($name, $itemsToSequence);
     }
diff --git a/PHPSkills/FactorGraphs/Message.php b/PHPSkills/FactorGraphs/Message.php
index 47d217e..c318dcf 100644
--- a/PHPSkills/FactorGraphs/Message.php
+++ b/PHPSkills/FactorGraphs/Message.php
@@ -12,11 +12,16 @@ class Message
         $this->_value = $value;
     }
 
-    public function getValue()
+    public function& getValue()
     {
         return $this->_value;
     }
 
+    public function setValue(&$value)
+    {
+        $this->_value = &$value;
+    }
+
     public function __toString()
     {
         return $this->_name; 
diff --git a/PHPSkills/HashMap.php b/PHPSkills/HashMap.php
index dc4f3c1..c1e4446 100644
--- a/PHPSkills/HashMap.php
+++ b/PHPSkills/HashMap.php
@@ -9,7 +9,8 @@ class HashMap
 
     public function getValue($key)
     {
-        return $this->_hashToValue[self::getHash($key)];
+        $hash = self::getHash($key);
+        return $this->_hashToValue[$hash];
     }
 
     public function setValue($key, $value)
diff --git a/PHPSkills/Numerics/GaussianDistribution.php b/PHPSkills/Numerics/GaussianDistribution.php
index 6c0dacb..55cf685 100644
--- a/PHPSkills/Numerics/GaussianDistribution.php
+++ b/PHPSkills/Numerics/GaussianDistribution.php
@@ -115,7 +115,7 @@ class GaussianDistribution
     // Computes the absolute difference between two Gaussians
     public static function subtract(GaussianDistribution $left, GaussianDistribution $right)
     {
-        return absoluteDifference($left, $right);
+        return GaussianDistribution::absoluteDifference($left, $right);
     }
     
     public static function logProductNormalization(GaussianDistribution $left, GaussianDistribution $right)
diff --git a/PHPSkills/TrueSkill/Factors/GaussianFactor.php b/PHPSkills/TrueSkill/Factors/GaussianFactor.php
index d9d7567..98d98db 100644
--- a/PHPSkills/TrueSkill/Factors/GaussianFactor.php
+++ b/PHPSkills/TrueSkill/Factors/GaussianFactor.php
@@ -25,7 +25,7 @@ abstract class GaussianFactor extends Factor
         $marginal = &$variable->getValue();
         $messageValue = &$message->getValue();
         $logZ = GaussianDistribution::logProductNormalization($marginal, $messageValue);
-        $variable->setValue($marginal*$messageValue);
+        $variable->setValue(GaussianDistribution::multiply($marginal, $messageValue));
         return $logZ;
     }
 
diff --git a/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php b/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php
index be2e41e..a30dedd 100644
--- a/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php
+++ b/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php
@@ -66,8 +66,8 @@ class GaussianLikelihoodFactor extends GaussianFactor
 
     public function updateMessageIndex($messageIndex)
     {
-        $messages = $this->getMessages();
-        $vars = $this->getVariables();
+        $messages = &$this->getMessages();
+        $vars = &$this->getVariables();
 
         switch ($messageIndex)
         {
diff --git a/PHPSkills/TrueSkill/TrueSkillFactorGraph.php b/PHPSkills/TrueSkill/TrueSkillFactorGraph.php
index dbff5fe..9330e09 100644
--- a/PHPSkills/TrueSkill/TrueSkillFactorGraph.php
+++ b/PHPSkills/TrueSkill/TrueSkillFactorGraph.php
@@ -91,7 +91,7 @@ class TrueSkillFactorGraph extends FactorGraph
 
         foreach ($this->_layers as $currentLayer)
         {
-            foreach ($currentLayer->getFactors() as $currentFactor)
+            foreach ($currentLayer->getLocalFactors() as $currentFactor)
             {
                 $factorList->addFactor($currentFactor);
             }
@@ -107,10 +107,10 @@ class TrueSkillFactorGraph extends FactorGraph
 
         foreach ($this->_layers as $currentLayer)
         {
-            $currentPriorSchedule = &$currentLayer->createPriorSchedule();
+            $currentPriorSchedule = $currentLayer->createPriorSchedule();
             if ($currentPriorSchedule != null)
             {
-                $fullSchedule[] = &$currentPriorSchedule;
+                $fullSchedule[] = $currentPriorSchedule;
             }
         }