diff --git a/PHPSkills/Elo/FideEloCalculator.php b/PHPSkills/Elo/FideEloCalculator.php
index da3cbdf..adc2fd9 100644
--- a/PHPSkills/Elo/FideEloCalculator.php
+++ b/PHPSkills/Elo/FideEloCalculator.php
@@ -5,7 +5,7 @@ namespace Moserware\Skills\Elo;
 require_once(dirname(__FILE__) . "/TwoPlayerEloCalculator.php");
 require_once(dirname(__FILE__) . "/FideKFactor.php");
 
-/** Including ELO's scheme as a simple comparison. 
+/** Including Elo's scheme as a simple comparison.
  *  See http://en.wikipedia.org/wiki/Elo_rating_system#Theory
  *  for more details
  */
diff --git a/PHPSkills/FactorGraphs/Factor.php b/PHPSkills/FactorGraphs/Factor.php
index 427c517..f7e3157 100644
--- a/PHPSkills/FactorGraphs/Factor.php
+++ b/PHPSkills/FactorGraphs/Factor.php
@@ -50,7 +50,7 @@ abstract class Factor
     {
         Guard::argumentIsValidIndex($messageIndex, count($this->_messages), "messageIndex");
         $message = &$this->_messages[$messageIndex];
-        $variable = &$this->_messageToVariableBinding->getValue($this->_messages[$messageIndex]);
+        $variable = &$this->_messageToVariableBinding->getValue($message);
         return $this->updateMessageVariable($message, $variable);
     }
 
@@ -62,7 +62,8 @@ abstract class Factor
     /// Resets the marginal of the variables a factor is connected to
     public function resetMarginals()
     {
-        foreach ($this->_messageToVariableBinding->getAllValues() as $currentVariable)
+        $allValues = &$this->_messageToVariableBinding->getAllValues();
+        foreach ($allValues as $currentVariable)
         {
             $currentVariable->resetToPrior();
         }
@@ -73,21 +74,21 @@ abstract class Factor
     {
         Guard::argumentIsValidIndex($messageIndex, count($this->_messages), "messageIndex");
 
-        $message = $this->_messages[$messageIndex];
-        $variable = $this->_messageToVariableBinding->getValue($message);
+        $message = &$this->_messages[$messageIndex];
+        $variable = &$this->_messageToVariableBinding->getValue($message);
         return $this->sendMessageVariable($message, $variable);
     }
 
     protected abstract function sendMessageVariable(Message &$message, Variable &$variable);
 
-    public abstract function createVariableToMessageBinding(Variable &$variable);
+    public abstract function &createVariableToMessageBinding(Variable &$variable);
 
-    protected function createVariableToMessageBindingWithMessage(Variable &$variable, Message &$message)
+    protected function &createVariableToMessageBindingWithMessage(Variable &$variable, Message &$message)
     {
         $index = count($this->_messages);
-        $this->_messages[] = $message;        
+        $this->_messages[] = &$message;
         $this->_messageToVariableBinding->setValue($message, $variable);
-        $this->_variables[] = $variable;
+        $this->_variables[] = &$variable;
         return $message;
     }
 
diff --git a/PHPSkills/FactorGraphs/FactorGraph.php b/PHPSkills/FactorGraphs/FactorGraph.php
index b8fe431..3685f1f 100644
--- a/PHPSkills/FactorGraphs/FactorGraph.php
+++ b/PHPSkills/FactorGraphs/FactorGraph.php
@@ -1,16 +1,19 @@
 <?php
 namespace Moserware\Skills\FactorGraphs;
 
+require_once(dirname(__FILE__) . "/VariableFactory.php");
+
 class FactorGraph
 {
     private $_variableFactory;
 
     public function &getVariableFactory()
     {
-        return $this->_variableFactory;
+        $factory = &$this->_variableFactory;
+        return $factory;
     }
 
-    public function setVariableFactory(&$factory)
+    public function setVariableFactory(VariableFactory &$factory)
     {
         $this->_variableFactory = &$factory;
     }
diff --git a/PHPSkills/FactorGraphs/FactorGraphLayer.php b/PHPSkills/FactorGraphs/FactorGraphLayer.php
index ec0e93a..74036b6 100644
--- a/PHPSkills/FactorGraphs/FactorGraphLayer.php
+++ b/PHPSkills/FactorGraphs/FactorGraphLayer.php
@@ -1,6 +1,7 @@
 <?php
 namespace Moserware\Skills\FactorGraphs;
 
+require_once(dirname(__FILE__) . "/Factor.php");
 require_once(dirname(__FILE__) . "/FactorGraph.php");
 require_once(dirname(__FILE__) . "/Schedule.php");
 
@@ -18,24 +19,28 @@ abstract class FactorGraphLayer
 
     protected function &getInputVariablesGroups()
     {
-        return $this->_inputVariablesGroups;        
+        $inputVariablesGroups = &$this->_inputVariablesGroups;
+        return $inputVariablesGroups;
     }
 
     // HACK
 
     public function &getParentFactorGraph()
     {
-        return $this->_parentFactorGraph;
+        $parentFactorGraph = &$this->_parentFactorGraph;
+        return $parentFactorGraph;
     }
 
     public function &getOutputVariablesGroups()
     {
-        return $this->_outputVariablesGroups;        
+        $outputVariablesGroups = &$this->_outputVariablesGroups;
+        return $outputVariablesGroups;
     }
 
     public function &getLocalFactors()
     {
-        return $this->_localFactors;        
+        $localFactors = &$this->_localFactors;
+        return $localFactors;
     }
 
     public function setInputVariablesGroups(&$value)
@@ -43,12 +48,12 @@ abstract class FactorGraphLayer
         $this->_inputVariablesGroups = $value;
     }
 
-    protected function scheduleSequence($itemsToSequence, $name)
+    protected function scheduleSequence(array $itemsToSequence, $name)
     {
         return new ScheduleSequence($name, $itemsToSequence);
     }
 
-    protected function addLayerFactor(&$factor)
+    protected function addLayerFactor(Factor &$factor)
     {
         $this->_localFactors[] = $factor;
     }
diff --git a/PHPSkills/FactorGraphs/FactorList.php b/PHPSkills/FactorGraphs/FactorList.php
index 527591c..199f0ae 100644
--- a/PHPSkills/FactorGraphs/FactorList.php
+++ b/PHPSkills/FactorGraphs/FactorList.php
@@ -13,7 +13,8 @@ class FactorList
 
     public function getLogNormalization()
     {
-        foreach($this->_list as $currentFactor)
+        $list = &$this->_list;
+        foreach($list as &$currentFactor)
         {
             $currentFactor->resetMarginals();
         }
@@ -36,7 +37,7 @@ class FactorList
 
         $sumLogS = 0;
 
-        foreach($this->_list as $currentFactor)
+        foreach($list as &$currentFactor)
         {
             $sumLogS = $sumLogS + $currentFactor->getLogNormalization();
         }
@@ -49,7 +50,7 @@ class FactorList
         return count($this->_list);
     }
 
-    public function addFactor(Factor &$factor)
+    public function &addFactor(Factor &$factor)
     {
         $this->_list[] = $factor;
         return $factor;
diff --git a/PHPSkills/FactorGraphs/Message.php b/PHPSkills/FactorGraphs/Message.php
index c318dcf..7d49e80 100644
--- a/PHPSkills/FactorGraphs/Message.php
+++ b/PHPSkills/FactorGraphs/Message.php
@@ -6,7 +6,7 @@ class Message
     private $_name;    
     private $_value;
 
-    public function __construct($value = null, $name = null)
+    public function __construct(&$value = null, $name = null)
     {
         $this->_name = $name;        
         $this->_value = $value;
@@ -14,7 +14,8 @@ class Message
 
     public function& getValue()
     {
-        return $this->_value;
+        $value = &$this->_value;
+        return $value;
     }
 
     public function setValue(&$value)
diff --git a/PHPSkills/FactorGraphs/Schedule.php b/PHPSkills/FactorGraphs/Schedule.php
index 4ac941a..a144ef8 100644
--- a/PHPSkills/FactorGraphs/Schedule.php
+++ b/PHPSkills/FactorGraphs/Schedule.php
@@ -1,6 +1,8 @@
 <?php
 namespace Moserware\Skills\FactorGraphs;
 
+require_once(dirname(__FILE__) . "/Factor.php");
+
 abstract class Schedule
 {
     private $_name;
@@ -23,7 +25,7 @@ class ScheduleStep extends Schedule
     private $_factor;
     private $_index;
 
-    public function __construct($name, $factor, $index)
+    public function __construct($name, Factor &$factor, $index)
     {
         parent::__construct($name);
         $this->_factor = $factor;
@@ -41,7 +43,7 @@ class ScheduleSequence extends Schedule
 {
     private $_schedules;
 
-    public function __construct($name, $schedules)
+    public function __construct($name, array $schedules)
     {
         parent::__construct($name);
         $this->_schedules = $schedules;
@@ -51,7 +53,8 @@ class ScheduleSequence extends Schedule
     {
         $maxDelta = 0;
 
-        foreach ($this->_schedules as $currentSchedule)
+        $schedules = &$this->_schedules;
+        foreach ($schedules as &$currentSchedule)
         {
             $maxDelta = max($currentSchedule->visit($depth + 1, $maxDepth), $maxDelta);
         }
@@ -65,7 +68,7 @@ class ScheduleLoop extends Schedule
     private $_maxDelta;
     private $_scheduleToLoop;
 
-    public function __construct($name, Schedule $scheduleToLoop, $maxDelta)
+    public function __construct($name, Schedule &$scheduleToLoop, $maxDelta)
     {
         parent::__construct($name);
         $this->_scheduleToLoop = $scheduleToLoop;
diff --git a/PHPSkills/FactorGraphs/Variable.php b/PHPSkills/FactorGraphs/Variable.php
index bad9cbb..a936916 100644
--- a/PHPSkills/FactorGraphs/Variable.php
+++ b/PHPSkills/FactorGraphs/Variable.php
@@ -7,7 +7,7 @@ class Variable
     private $_prior;
     private $_value;
 
-    public function __construct($name, $prior)
+    public function __construct($name, &$prior)
     {
         $this->_name = "Variable[" . $name . "]";
         $this->_prior = $prior;
@@ -16,7 +16,8 @@ class Variable
 
     public function &getValue()
     {
-        return $this->_value;
+        $value = &$this->_value;
+        return $value;
     }
 
     public function setValue(&$value)
@@ -42,12 +43,12 @@ class DefaultVariable extends Variable
         parent::__construct("Default", null);
     }
 
-    public function getValue()
+    public function &getValue()
     {
         return null;
     }
 
-    public function setValue($value)
+    public function setValue(&$value)
     {
         throw new Exception();
     }
@@ -56,7 +57,7 @@ class DefaultVariable extends Variable
 class KeyedVariable extends Variable
 {
     private $_key;
-    public function __construct($key, $name, $prior)
+    public function __construct($key, $name, &$prior)
     {
         parent::__construct($name, $prior);
         $this->_key = $key;
diff --git a/PHPSkills/FactorGraphs/VariableFactory.php b/PHPSkills/FactorGraphs/VariableFactory.php
index 3d72f8e..12061f2 100644
--- a/PHPSkills/FactorGraphs/VariableFactory.php
+++ b/PHPSkills/FactorGraphs/VariableFactory.php
@@ -14,14 +14,14 @@ class VariableFactory
         $this->_variablePriorInitializer = &$variablePriorInitializer;
     }
 
-    public function createBasicVariable($name)
+    public function &createBasicVariable($name)
     {
         $initializer = $this->_variablePriorInitializer;
         $newVar = new Variable($name, $initializer());
         return $newVar;
     }
 
-    public function createKeyedVariable($key, $name)
+    public function &createKeyedVariable(&$key, $name)
     {
         $initializer = $this->_variablePriorInitializer;
         $newVar = new KeyedVariable($key, $name, $initializer());
diff --git a/PHPSkills/HashMap.php b/PHPSkills/HashMap.php
index c1e4446..efc3153 100644
--- a/PHPSkills/HashMap.php
+++ b/PHPSkills/HashMap.php
@@ -7,10 +7,11 @@ class HashMap
     private $_hashToValue = array();
     private $_hashToKey = array();
 
-    public function getValue($key)
+    public function &getValue($key)
     {
         $hash = self::getHash($key);
-        return $this->_hashToValue[$hash];
+        $hashValue = &$this->_hashToValue[$hash];
+        return $hashValue;
     }
 
     public function setValue($key, $value)
diff --git a/PHPSkills/Numerics/BasicMath.php b/PHPSkills/Numerics/BasicMath.php
index 37f8a1a..cac534c 100644
--- a/PHPSkills/Numerics/BasicMath.php
+++ b/PHPSkills/Numerics/BasicMath.php
@@ -15,7 +15,7 @@ function square($x)
     return $x * $x;
 }
 
-function sum($itemsToSum, $funcName )
+function sum(array $itemsToSum, $funcName )
 {
     $mappedItems = array_map($funcName, $itemsToSum);
     return array_sum($mappedItems);
diff --git a/PHPSkills/Numerics/GaussianDistribution.php b/PHPSkills/Numerics/GaussianDistribution.php
index 55cf685..406f9a3 100644
--- a/PHPSkills/Numerics/GaussianDistribution.php
+++ b/PHPSkills/Numerics/GaussianDistribution.php
@@ -30,8 +30,25 @@ class GaussianDistribution
         $this->_mean = $mean;        
         $this->_standardDeviation = $standardDeviation;
         $this->_variance = square($standardDeviation);
-        $this->_precision = 1.0/$this->_variance;
-        $this->_precisionMean = $this->_precision*$this->_mean;
+
+        if($this->_variance != 0)
+        {
+            $this->_precision = 1.0/$this->_variance;
+            $this->_precisionMean = $this->_precision*$this->_mean;
+        }
+        else
+        {
+            $this->_precision = \INF;
+
+            if($this->_mean == 0)
+            {
+                $this->_precisionMean = 0;
+            }
+            else
+            {
+                $this->_precisionMean = \INF;
+            }
+        }
     }    
     
     public function getMean()
diff --git a/PHPSkills/Player.php b/PHPSkills/Player.php
index 9819101..76d0f46 100644
--- a/PHPSkills/Player.php
+++ b/PHPSkills/Player.php
@@ -38,8 +38,9 @@ class Player implements ISupportPartialPlay, ISupportPartialUpdate
     /// <summary>
     /// The identifier for the player, such as a name.
     /// </summary>
-    public function getId()
+    public function &getId()
     {
+        $id = &$this->_Id;
         return $this->_Id;
     }
 
diff --git a/PHPSkills/RatingContainer.php b/PHPSkills/RatingContainer.php
index 456712a..d1e06ff 100644
--- a/PHPSkills/RatingContainer.php
+++ b/PHPSkills/RatingContainer.php
@@ -12,9 +12,10 @@ class RatingContainer
         $this->_playerToRating = new HashMap();
     }
 
-    public function getRating($player)
+    public function &getRating($player)
     {
-        return $this->_playerToRating->getValue($player);
+        $rating = &$this->_playerToRating->getValue($player);
+        return $rating;
     }
 
     public function setRating($player, $rating)
diff --git a/PHPSkills/Team.php b/PHPSkills/Team.php
index a4e6637..c33343e 100644
--- a/PHPSkills/Team.php
+++ b/PHPSkills/Team.php
@@ -5,7 +5,7 @@ require_once(dirname(__FILE__) . '/RatingContainer.php');
 
 class Team extends RatingContainer
 {
-    public function __construct($player = null, $rating = null)
+    public function __construct(&$player = null, &$rating = null)
     {
         parent::__construct();
         
@@ -15,7 +15,7 @@ class Team extends RatingContainer
         }
     }
 
-    public function addPlayer($player, $rating)
+    public function addPlayer(&$player, &$rating)
     {
         $this->setRating($player, $rating);
         return $this;
diff --git a/PHPSkills/Teams.php b/PHPSkills/Teams.php
index a34fd74..b55e8b5 100644
--- a/PHPSkills/Teams.php
+++ b/PHPSkills/Teams.php
@@ -8,7 +8,7 @@ class Teams
         $args = \func_get_args();
         $result = array();
 
-        foreach ($args as $currentTeam) {
+        foreach ($args as &$currentTeam) {
             $result[] = $currentTeam;
         }
 
diff --git a/PHPSkills/TrueSkill/Factors/GaussianFactor.php b/PHPSkills/TrueSkill/Factors/GaussianFactor.php
index 98d98db..62d1ed8 100644
--- a/PHPSkills/TrueSkill/Factors/GaussianFactor.php
+++ b/PHPSkills/TrueSkill/Factors/GaussianFactor.php
@@ -29,12 +29,13 @@ abstract class GaussianFactor extends Factor
         return $logZ;
     }
 
-    public function createVariableToMessageBinding(Variable &$variable)
+    public function &createVariableToMessageBinding(Variable &$variable)
     {
-        return parent::createVariableToMessageBindingWithMessage($variable,
+        $binding = &parent::createVariableToMessageBindingWithMessage($variable,
                                                       new Message(
                                                           GaussianDistribution::fromPrecisionMean(0, 0),
                                                           sprintf("message from %s to %s", $this, $variable)));
+        return $binding;
     }
 }
 
diff --git a/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php b/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php
index a30dedd..f5982c7 100644
--- a/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php
+++ b/PHPSkills/TrueSkill/Factors/GaussianLikelihoodFactor.php
@@ -28,8 +28,8 @@ class GaussianLikelihoodFactor extends GaussianFactor
 
     public function getLogNormalization()
     {
-        $vars = $this->getVariables();
-        $messages = $this->getMessages();
+        $vars = &$this->getVariables();
+        $messages = &$this->getMessages();
 
         return GaussianDistribution::logRatioNormalization(
                 $vars[0]->getValue(),
diff --git a/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php b/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php
index 03a9729..d4921b8 100644
--- a/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php
+++ b/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php
@@ -118,7 +118,7 @@ class GaussianWeightedSumFactor extends GaussianFactor
 
         $this->createVariableToMessageBinding($sumVariable);
 
-        foreach ($variablesToSum as $currentVariable)
+        foreach ($variablesToSum as &$currentVariable)
         {
             $this->createVariableToMessageBinding($currentVariable);
         }
diff --git a/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php b/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php
index 40ec599..15ffc22 100644
--- a/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php
+++ b/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php
@@ -33,8 +33,8 @@ class IteratedTeamDifferencesInnerLayer extends TrueSkillFactorGraphLayer
         $this->_TeamPerformancesToTeamPerformanceDifferencesLayer->setInputVariablesGroups($inputVariablesGroups);
         $this->_TeamPerformancesToTeamPerformanceDifferencesLayer->buildLayer();
 
-        $this->_TeamDifferencesComparisonLayer->setInputVariablesGroups(
-            $this->_TeamPerformancesToTeamPerformanceDifferencesLayer->getOutputVariablesGroups());
+        $teamDifferencesOutputVariablesGroups = &$this->_TeamPerformancesToTeamPerformanceDifferencesLayer->getOutputVariablesGroups();
+        $this->_TeamDifferencesComparisonLayer->setInputVariablesGroups($teamDifferencesOutputVariablesGroups);
         $this->_TeamDifferencesComparisonLayer->buildLayer();
     }
 
@@ -65,10 +65,10 @@ class IteratedTeamDifferencesInnerLayer extends TrueSkillFactorGraphLayer
                     $loop,
                     new ScheduleStep(
                         "teamPerformanceToPerformanceDifferenceFactors[0] @ 1",
-                        $localFactors[0], 1),
+                        &$localFactors[0], 1),
                     new ScheduleStep(
                         sprintf("teamPerformanceToPerformanceDifferenceFactors[teamTeamDifferences = %d - 1] @ 2", $totalTeamDifferences),
-                        $localFactors[$totalTeamDifferences - 1], 2)
+                        &$localFactors[$totalTeamDifferences - 1], 2)
                 )
             );
 
@@ -83,11 +83,11 @@ class IteratedTeamDifferencesInnerLayer extends TrueSkillFactorGraphLayer
         $itemsToSequence = array(
                     new ScheduleStep(
                         "send team perf to perf differences",
-                        $teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[0],
+                        &$teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[0],
                         0),
                     new ScheduleStep(
                         "send to greater than or within factor",
-                        $teamDifferencesComparisonLayerLocalFactors[0],
+                        &$teamDifferencesComparisonLayerLocalFactors[0],
                         0)
                 );
 
@@ -112,16 +112,16 @@ class IteratedTeamDifferencesInnerLayer extends TrueSkillFactorGraphLayer
                     array(
                             new ScheduleStep(
                                 sprintf("team perf to perf diff %d", $i),
-                                $teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$i], 0),
+                                &$teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$i], 0),
                             new ScheduleStep(
                                 sprintf("greater than or within result factor %d", $i),
-                                $teamDifferencesComparisonLayerLocalFactors[$i], 0),
+                                &$teamDifferencesComparisonLayerLocalFactors[$i], 0),
                             new ScheduleStep(
                                 sprintf("team perf to perf diff factors [%d], 2", $i),
-                                $teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$i], 2)
+                                &$teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$i], 2)
                         ), sprintf("current forward schedule piece %d", $i));
 
-            $forwardScheduleList[] = &$currentForwardSchedulePiece;
+            $forwardScheduleList[] = $currentForwardSchedulePiece;
         }
 
         $forwardSchedule = new ScheduleSequence("forward schedule", $forwardScheduleList);
@@ -133,20 +133,24 @@ class IteratedTeamDifferencesInnerLayer extends TrueSkillFactorGraphLayer
             $teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors = &$this->_TeamPerformancesToTeamPerformanceDifferencesLayer->getLocalFactors();
             $teamDifferencesComparisonLayerLocalFactors = &$this->_TeamDifferencesComparisonLayer->getLocalFactors();
 
+            $differencesFactor = &$teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$totalTeamDifferences - 1 - $i];
+            $comparisonFactor = &$teamDifferencesComparisonLayerLocalFactors[$totalTeamDifferences - 1 - $i];
+            $performancesToDifferencesFactor = &$teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$totalTeamDifferences - 1 - $i];
+
             $currentBackwardSchedulePiece = new ScheduleSequence(
                 "current backward schedule piece",
                 array(
                         new ScheduleStep(
                             sprintf("teamPerformanceToPerformanceDifferenceFactors[totalTeamDifferences - 1 - %d] @ 0", $i),
-                            $teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$totalTeamDifferences - 1 - $i], 0),
+                            $differencesFactor, 0),
                         new ScheduleStep(
                             sprintf("greaterThanOrWithinResultFactors[totalTeamDifferences - 1 - %d] @ 0", $i),
-                            $teamDifferencesComparisonLayerLocalFactors[$totalTeamDifferences - 1 - $i], 0),
+                            $comparisonFactor, 0),
                         new ScheduleStep(
                             sprintf("teamPerformanceToPerformanceDifferenceFactors[totalTeamDifferences - 1 - %d] @ 1", $i),
-                            $teamPerformancesToTeamPerformanceDifferencesLayerLocalFactors[$totalTeamDifferences - 1 - $i], 1)
+                            $performancesToDifferencesFactor, 1)
                 ));
-            $backwardScheduleList[] = &$currentBackwardSchedulePiece;
+            $backwardScheduleList[] = $currentBackwardSchedulePiece;
         }
 
         $backwardSchedule = new ScheduleSequence("backward schedule", $backwardScheduleList);
diff --git a/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php b/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php
index 71080fc..90c8fb0 100644
--- a/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php
+++ b/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php
@@ -39,14 +39,17 @@ class PlayerPerformancesToTeamPerformancesLayer extends TrueSkillFactorGraphLaye
 
     public function createPriorSchedule()
     {
-        return $this->scheduleSequence(
-                array_map(
-                        function($weightedSumFactor)
-                        {
-                            return new ScheduleStep("Perf to Team Perf Step", $weightedSumFactor, 0);
-                        },
-                        $this->getLocalFactors()),
-                "all player perf to team perf schedule");
+        $localFactors = &$this->getLocalFactors();
+
+        $sequence = &$this->scheduleSequence(
+                                            array_map(
+                                                    function($weightedSumFactor)
+                                                    {
+                                                        return new ScheduleStep("Perf to Team Perf Step", $weightedSumFactor, 0);
+                                                    },
+                                                    $localFactors),
+                                            "all player perf to team perf schedule");
+        return $sequence;
     }
 
     protected function createPlayerToTeamSumFactor(&$teamMembers, &$sumVariable)
@@ -69,7 +72,8 @@ class PlayerPerformancesToTeamPerformancesLayer extends TrueSkillFactorGraphLaye
     {
         // BLOG
         $allFactors = array();
-        foreach($this->getLocalFactors() as $currentFactor)
+        $localFactors = &$this->getLocalFactors();
+        foreach($localFactors as &$currentFactor)
         {
             $numberOfMessages = $currentFactor->getNumberOfMessages();
             for($currentIteration = 1; $currentIteration < $numberOfMessages; $currentIteration++)
diff --git a/PHPSkills/TrueSkill/Layers/PlayerPriorValuesToSkillsLayer.php b/PHPSkills/TrueSkill/Layers/PlayerPriorValuesToSkillsLayer.php
index 1e0843a..617cbe0 100644
--- a/PHPSkills/TrueSkill/Layers/PlayerPriorValuesToSkillsLayer.php
+++ b/PHPSkills/TrueSkill/Layers/PlayerPriorValuesToSkillsLayer.php
@@ -1,15 +1,20 @@
 <?php
 namespace Moserware\Skills\TrueSkill\Layers;
 
+require_once(dirname(__FILE__) . "/../../Rating.php");
 require_once(dirname(__FILE__) . "/../../FactorGraphs/Schedule.php");
+require_once(dirname(__FILE__) . "/../../FactorGraphs/Variable.php");
 require_once(dirname(__FILE__) . "/../../Numerics/BasicMath.php");
 require_once(dirname(__FILE__) . "/../TrueSkillFactorGraph.php");
 require_once(dirname(__FILE__) . "/../Factors/GaussianPriorFactor.php");
 require_once(dirname(__FILE__) . "/TrueSkillFactorGraphLayer.php");
 
+use Moserware\Skills\Rating;
 use Moserware\Skills\FactorGraphs\ScheduleLoop;
 use Moserware\Skills\FactorGraphs\ScheduleSequence;
 use Moserware\Skills\FactorGraphs\ScheduleStep;
+use Moserware\Skills\FactorGraphs\Variable;
+use Moserware\Numerics\GaussianDistribution;
 use Moserware\Skills\TrueSkill\TrueSkillFactorGraph;
 use Moserware\Skills\TrueSkill\Factors\GaussianPriorFactor;
 
@@ -27,11 +32,13 @@ class PlayerPriorValuesToSkillsLayer extends TrueSkillFactorGraphLayer
 
     public function buildLayer()
     {
-        foreach ($this->_teams as $currentTeam)
+        $teams = &$this->_teams;
+        foreach ($teams as &$currentTeam)
         {
             $currentTeamSkills = array();
 
-            foreach ($currentTeam->getAllPlayers() as $currentTeamPlayer)
+            $currentTeamAllPlayers = &$currentTeam->getAllPlayers();
+            foreach ($currentTeamAllPlayers as &$currentTeamPlayer)
             {
                 $currentTeamPlayerRating = $currentTeam->getRating($currentTeamPlayer);
                 $playerSkill = $this->createSkillOutputVariable($currentTeamPlayer);
@@ -47,17 +54,18 @@ class PlayerPriorValuesToSkillsLayer extends TrueSkillFactorGraphLayer
 
     public function createPriorSchedule()
     {
+        $localFactors = &$this->getLocalFactors();
         return $this->scheduleSequence(
                 array_map(
-                        function($prior)
+                        function(&$prior)
                         {
                             return new ScheduleStep("Prior to Skill Step", $prior, 0);
                         },
-                        $this->getLocalFactors()),
+                        $localFactors),
                  "All priors");
     }
 
-    private function createPriorFactor(&$player, &$priorRating, &$skillsVariable)
+    private function createPriorFactor(&$player, Rating &$priorRating, Variable &$skillsVariable)
     {
         return new GaussianPriorFactor($priorRating->getMean(),
                                        square($priorRating->getStandardDeviation()) +
@@ -67,8 +75,8 @@ class PlayerPriorValuesToSkillsLayer extends TrueSkillFactorGraphLayer
 
     private function createSkillOutputVariable($key)
     {
-        $parentFactorGraph = $this->getParentFactorGraph();
-        $variableFactory = $parentFactorGraph->getVariableFactory();
+        $parentFactorGraph = &$this->getParentFactorGraph();
+        $variableFactory = &$parentFactorGraph->getVariableFactory();
         return $variableFactory->createKeyedVariable($key, $key . "'s skill");
     }
 }
diff --git a/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php b/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php
index 4478616..19adef9 100644
--- a/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php
+++ b/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php
@@ -2,12 +2,14 @@
 namespace Moserware\Skills\TrueSkill\Layers;
 
 require_once(dirname(__FILE__) . "/../../FactorGraphs/Schedule.php");
+require_once(dirname(__FILE__) . "/../../FactorGraphs/Variable.php");
 require_once(dirname(__FILE__) . "/../../Numerics/BasicMath.php");
 require_once(dirname(__FILE__) . "/../TrueSkillFactorGraph.php");
 require_once(dirname(__FILE__) . "/../Factors/GaussianLikelihoodFactor.php");
 require_once(dirname(__FILE__) . "/TrueSkillFactorGraphLayer.php");
 
 use Moserware\Skills\FactorGraphs\ScheduleStep;
+use Moserware\Skills\FactorGraphs\KeyedVariable;
 use Moserware\Skills\TrueSkill\TrueSkillFactorGraph;
 use Moserware\Skills\TrueSkill\Factors\GaussianLikelihoodFactor;
 
@@ -20,7 +22,10 @@ class PlayerSkillsToPerformancesLayer extends TrueSkillFactorGraphLayer
 
     public function buildLayer()
     {
-        foreach ($this->getInputVariablesGroups() as $currentTeam)
+        $inputVariablesGroups = &$this->getInputVariablesGroups();
+        $outputVariablesGroups = &$this->getOutputVariablesGroups();
+
+        foreach ($inputVariablesGroups as &$currentTeam)
         {
             $currentTeamPlayerPerformances = array();
 
@@ -30,14 +35,13 @@ class PlayerSkillsToPerformancesLayer extends TrueSkillFactorGraphLayer
                 $newLikelihoodFactor = $this->createLikelihood($playerSkillVariable, $playerPerformance);
                 $this->addLayerFactor($newLikelihoodFactor);
                 $currentTeamPlayerPerformances[] = $playerPerformance;
-            }
+            }            
             
-            $outputVariablesGroups = &$this->getOutputVariablesGroups();
             $outputVariablesGroups[] = $currentTeamPlayerPerformances;
         }
     }
 
-    private function createLikelihood(&$playerSkill, &$playerPerformance)
+    private function createLikelihood(KeyedVariable &$playerSkill, KeyedVariable &$playerPerformance)
     {
         return new GaussianLikelihoodFactor(square($this->getParentFactorGraph()->getGameInfo()->getBeta()), $playerPerformance, $playerSkill);
     }
@@ -49,25 +53,27 @@ class PlayerSkillsToPerformancesLayer extends TrueSkillFactorGraphLayer
 
     public function createPriorSchedule()
     {
+        $localFactors = &$this->getLocalFactors();
         return $this->scheduleSequence(
                 array_map(
                         function($likelihood)
                         {
                             return new ScheduleStep("Skill to Perf step", $likelihood, 0);
                         },
-                        $this->getLocalFactors()),
+                        $localFactors),
                 "All skill to performance sending");
     }
 
     public function createPosteriorSchedule()
     {
+        $localFactors = &$this->getLocalFactors();
         return $this->scheduleSequence(
                 array_map(
                         function($likelihood)
                         {
                             return new ScheduleStep("name", $likelihood, 1);                    
                         },
-                        $this->getLocalFactors()),
+                        $localFactors),
                 "All skill to performance sending");
     }
 }
diff --git a/PHPSkills/TrueSkill/Layers/TeamDifferencesComparisonLayer.php b/PHPSkills/TrueSkill/Layers/TeamDifferencesComparisonLayer.php
index 8cf2993..c66bb67 100644
--- a/PHPSkills/TrueSkill/Layers/TeamDifferencesComparisonLayer.php
+++ b/PHPSkills/TrueSkill/Layers/TeamDifferencesComparisonLayer.php
@@ -21,7 +21,7 @@ class TeamDifferencesComparisonLayer extends TrueSkillFactorGraphLayer
     {
         parent::__construct($parentGraph);
         $this->_teamRanks = $teamRanks;
-        $gameInfo = $this->getParentFactorGraph()->getGameInfo();
+        $gameInfo = &$this->getParentFactorGraph()->getGameInfo();
         $this->_epsilon = DrawMargin::getDrawMarginFromDrawProbability($gameInfo->getDrawProbability(), $gameInfo->getBeta());
     }
 
@@ -33,7 +33,7 @@ class TeamDifferencesComparisonLayer extends TrueSkillFactorGraphLayer
         for ($i = 0; $i < $inputVarGroupsCount; $i++)
         {
             $isDraw = ($this->_teamRanks[$i] == $this->_teamRanks[$i + 1]);
-            $teamDifference = $inputVarGroups[$i][0];
+            $teamDifference = &$inputVarGroups[$i][0];
 
             $factor =
                 $isDraw
diff --git a/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php b/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php
index 132c23f..a5a6e38 100644
--- a/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php
+++ b/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php
@@ -22,18 +22,18 @@ class TeamPerformancesToTeamPerformanceDifferencesLayer extends TrueSkillFactorG
     {
         $inputVariablesGroups = &$this->getInputVariablesGroups();
         $inputVariablesGroupsCount = count($inputVariablesGroups);
+        $outputVariablesGroup = &$this->getOutputVariablesGroups();
 
         for ($i = 0; $i < $inputVariablesGroupsCount - 1; $i++)
         {
-            $strongerTeam = $inputVariablesGroups[$i][0];
-            $weakerTeam = $inputVariablesGroups[$i + 1][0];
+            $strongerTeam = &$inputVariablesGroups[$i][0];
+            $weakerTeam = &$inputVariablesGroups[$i + 1][0];
 
             $currentDifference = &$this->createOutputVariable();
             $newDifferencesFactor = $this->createTeamPerformanceToDifferenceFactor($strongerTeam, $weakerTeam, $currentDifference);
             $this->addLayerFactor($newDifferencesFactor);
 
-            // REVIEW: Does it make sense to have groups of one?
-            $outputVariablesGroup = &$this->getOutputVariablesGroups();
+            // REVIEW: Does it make sense to have groups of one?            
             $outputVariablesGroup[] = array($currentDifference);
         }
     }
diff --git a/PHPSkills/TrueSkill/TrueSkillFactorGraph.php b/PHPSkills/TrueSkill/TrueSkillFactorGraph.php
index 69f07fc..b000ef0 100644
--- a/PHPSkills/TrueSkill/TrueSkillFactorGraph.php
+++ b/PHPSkills/TrueSkill/TrueSkillFactorGraph.php
@@ -68,7 +68,8 @@ class TrueSkillFactorGraph extends FactorGraph
     {
         $lastOutput = null;
 
-        foreach ($this->_layers as $currentLayer)
+        $layers = &$this->_layers;
+        foreach ($layers as &$currentLayer)
         {
             if ($lastOutput != null)
             {
@@ -91,9 +92,11 @@ class TrueSkillFactorGraph extends FactorGraph
     {
         $factorList = new FactorList();
 
-        foreach ($this->_layers as $currentLayer)
+        $layers = &$this->_layers;
+        foreach ($layers as &$currentLayer)
         {
-            foreach ($currentLayer->getLocalFactors() as $currentFactor)
+            $localFactors = &$currentLayer->getLocalFactors();
+            foreach ($localFactors as &$currentFactor)
             {
                 $factorList->addFactor($currentFactor);
             }
@@ -107,7 +110,8 @@ class TrueSkillFactorGraph extends FactorGraph
     {
         $fullSchedule = array();
 
-        foreach ($this->_layers as $currentLayer)
+        $layers = &$this->_layers;
+        foreach ($layers as &$currentLayer)
         {
             $currentPriorSchedule = $currentLayer->createPriorSchedule();
             if ($currentPriorSchedule != null)
@@ -118,7 +122,7 @@ class TrueSkillFactorGraph extends FactorGraph
         
         $allLayersReverse = \array_reverse($this->_layers);
 
-        foreach ($allLayersReverse as $currentLayer)
+        foreach ($allLayersReverse as &$currentLayer)
         {
             $currentPosteriorSchedule = $currentLayer->createPosteriorSchedule();
             if ($currentPosteriorSchedule != null)
@@ -134,9 +138,10 @@ class TrueSkillFactorGraph extends FactorGraph
     {
         $result = new RatingContainer();
 
-        foreach ($this->_priorLayer->getOutputVariablesGroups() as $currentTeam)
+        $priorLayerOutputVariablesGroups = &$this->_priorLayer->getOutputVariablesGroups();
+        foreach ($priorLayerOutputVariablesGroups as &$currentTeam)
         {
-            foreach ($currentTeam as $currentPlayer)
+            foreach ($currentTeam as &$currentPlayer)
             {
                 $newRating = new Rating($currentPlayer->getValue()->getMean(),
                                         $currentPlayer->getValue()->getStandardDeviation());
diff --git a/PHPSkills/TrueSkill/TwoPlayerTrueSkillCalculator.php b/PHPSkills/TrueSkill/TwoPlayerTrueSkillCalculator.php
index dea523c..0e49026 100644
--- a/PHPSkills/TrueSkill/TwoPlayerTrueSkillCalculator.php
+++ b/PHPSkills/TrueSkill/TwoPlayerTrueSkillCalculator.php
@@ -2,6 +2,7 @@
 
 namespace Moserware\Skills\TrueSkill;
 
+require_once(dirname(__FILE__) . "/../GameInfo.php");
 require_once(dirname(__FILE__) . "/../Guard.php");
 require_once(dirname(__FILE__) . "/../PairwiseComparison.php");
 require_once(dirname(__FILE__) . "/../RankSorter.php");
@@ -17,6 +18,7 @@ require_once(dirname(__FILE__) . "/../Numerics/BasicMath.php");
 require_once(dirname(__FILE__) . "/DrawMargin.php");
 require_once(dirname(__FILE__) . "/TruncatedGaussianCorrectionFunctions.php");
 
+use Moserware\Skills\GameInfo;
 use Moserware\Skills\Guard;
 use Moserware\Skills\PairwiseComparison;
 use Moserware\Skills\RankSorter;
@@ -42,7 +44,7 @@ class TwoPlayerTrueSkillCalculator extends SkillCalculator
         parent::__construct(SkillCalculatorSupportedOptions::NONE, TeamsRange::exactly(2), PlayersRange::exactly(1));
     }
 
-    public function calculateNewRatings($gameInfo,
+    public function calculateNewRatings(GameInfo &$gameInfo,
                                         array $teams,
                                         array $teamRanks)
     {
@@ -82,7 +84,7 @@ class TwoPlayerTrueSkillCalculator extends SkillCalculator
         return $results;
     }
 
-    private static function calculateNewRating($gameInfo, $selfRating, $opponentRating, $comparison)
+    private static function calculateNewRating(GameInfo $gameInfo, Rating $selfRating, Rating $opponentRating, $comparison)
     {
         $drawMargin = DrawMargin::getDrawMarginFromDrawProbability($gameInfo->getDrawProbability(),
                                                                    $gameInfo->getBeta());
@@ -138,7 +140,7 @@ class TwoPlayerTrueSkillCalculator extends SkillCalculator
     }
 
     /// <inheritdoc/>
-    public function calculateMatchQuality($gameInfo, array $teams)
+    public function calculateMatchQuality(GameInfo $gameInfo, array $teams)
     {
         Guard::argumentNotNull($gameInfo, "gameInfo");
         $this->validateTeamCountAndPlayersCountPerTeam($teams);
diff --git a/PHPSkills/TrueSkill/TwoTeamTrueSkillCalculator.php b/PHPSkills/TrueSkill/TwoTeamTrueSkillCalculator.php
index 5ce33e4..cf742f4 100644
--- a/PHPSkills/TrueSkill/TwoTeamTrueSkillCalculator.php
+++ b/PHPSkills/TrueSkill/TwoTeamTrueSkillCalculator.php
@@ -47,7 +47,7 @@ class TwoTeamTrueSkillCalculator extends SkillCalculator
         parent::__construct(SkillCalculatorSupportedOptions::NONE, TeamsRange::exactly(2), PlayersRange::atLeast(1));
     }
 
-    public function calculateNewRatings($gameInfo,
+    public function calculateNewRatings(GameInfo $gameInfo,
                                         array $teams,
                                         array $teamRanks)
     {
@@ -164,7 +164,7 @@ class TwoTeamTrueSkillCalculator extends SkillCalculator
     }
 
     /// <inheritdoc/>
-    public function calculateMatchQuality($gameInfo, array $teams)
+    public function calculateMatchQuality(GameInfo $gameInfo, array $teams)
     {
         Guard::argumentNotNull($gameInfo, "gameInfo");
         $this->validateTeamCountAndPlayersCountPerTeam($teams);