diff --git a/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php b/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php
index 6aa657c..c96947e 100644
--- a/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php
+++ b/PHPSkills/TrueSkill/Factors/GaussianWeightedSumFactor.php
@@ -93,7 +93,7 @@ class GaussianWeightedSumFactor extends GaussianFactor
                 }
 
                 $currentWeights[$currentDestinationWeightIndex] = $currentWeight;
-                $currentWeightsSquared[$currentDestinationWeightIndex] = $currentWeight*currentWeight;
+                $currentWeightsSquared[$currentDestinationWeightIndex] = $currentWeight*$currentWeight;
 
                 $variableIndices[$currentDestinationWeightIndex + 1] = $currentWeightSourceIndex + 1;
                 $currentDestinationWeightIndex++;
diff --git a/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php b/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php
index 6554ea7..1ce6f87 100644
--- a/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php
+++ b/PHPSkills/TrueSkill/Layers/IteratedTeamDifferencesInnerLayer.php
@@ -29,7 +29,8 @@ class IteratedTeamDifferencesInnerLayer extends TrueSkillFactorGraphLayer
 
     public function buildLayer()
     {
-        $this->_TeamPerformancesToTeamPerformanceDifferencesLayer->setInputVariablesGroups($this->getInputVariablesGroups());
+        $inputVariablesGroups = $this->getInputVariablesGroups();
+        $this->_TeamPerformancesToTeamPerformanceDifferencesLayer->setInputVariablesGroups($inputVariablesGroups);
         $this->_TeamPerformancesToTeamPerformanceDifferencesLayer->buildLayer();
 
         $this->_TeamDifferencesComparisonLayer->setInputVariablesGroups(
diff --git a/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php b/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php
index 3f50550..7803a03 100644
--- a/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php
+++ b/PHPSkills/TrueSkill/Layers/PlayerPerformancesToTeamPerformancesLayer.php
@@ -27,12 +27,13 @@ class PlayerPerformancesToTeamPerformancesLayer extends TrueSkillFactorGraphLaye
     {
         foreach ($this->getInputVariablesGroups() as $currentTeam)
         {
-            $teamPerformance = $this->createOutputVariable($currentTeam);
-            $this->addLayerFactor($this->createPlayerToTeamSumFactor($currentTeam, $teamPerformance));
+            $teamPerformance = &$this->createOutputVariable($currentTeam);
+            $newSumFactor = $this->createPlayerToTeamSumFactor($currentTeam, $teamPerformance);
+            $this->addLayerFactor($newSumFactor);
 
             // REVIEW: Does it make sense to have groups of one?
-            $outputVariablesGroups = $this->getOutputVariablesGroups();
-            $outputVariablesGroups = $teamPerformance;
+            $outputVariablesGroups = &$this->getOutputVariablesGroups();
+            $outputVariablesGroups[] = array($teamPerformance);
         }
     }
 
diff --git a/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php b/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php
index 261fd2b..f9f8b45 100644
--- a/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php
+++ b/PHPSkills/TrueSkill/Layers/PlayerSkillsToPerformancesLayer.php
@@ -27,7 +27,8 @@ class PlayerSkillsToPerformancesLayer extends TrueSkillFactorGraphLayer
             foreach ($currentTeam as $playerSkillVariable)
             {
                 $playerPerformance = $this->createOutputVariable($playerSkillVariable->getKey());
-                $this->addLayerFactor($this->createLikelihood($playerSkillVariable, $playerPerformance));
+                $newLikelihoodFactor = $this->createLikelihood($playerSkillVariable, $playerPerformance);
+                $this->addLayerFactor($newLikelihoodFactor);
                 $currentTeamPlayerPerformances[] = &$playerPerformance;
             }
             
diff --git a/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php b/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php
index 9963224..75803ae 100644
--- a/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php
+++ b/PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php
@@ -20,16 +20,17 @@ class TeamPerformancesToTeamPerformanceDifferencesLayer extends TrueSkillFactorG
 
     public function buildLayer()
     {
-        $inputVariablesGroup = $this->getInputVariablesGroups();
-        $inputVariablesGroupCount = count($inputVariablesGroup);
+        $inputVariablesGroups = &$this->getInputVariablesGroups();
+        $inputVariablesGroupsCount = count($inputVariablesGroups);
 
-        for ($i = 0; $i < $inputVariablesGroupCount - 1; $i++)
+        for ($i = 0; $i < $inputVariablesGroupsCount - 1; $i++)
         {
             $strongerTeam = $inputVariablesGroups[$i][0];
             $weakerTeam = $inputVariablesGroups[$i + 1][0];
 
             $currentDifference = $this->createOutputVariable();
-            $this->addLayerFactor($this->createTeamPerformanceToDifferenceFactor($strongerTeam, $weakerTeam, currentDifference));
+            $newDifferencesFactor = $this->createTeamPerformanceToDifferenceFactor($strongerTeam, $weakerTeam, $currentDifference);
+            $this->addLayerFactor($newDifferencesFactor);
 
             // REVIEW: Does it make sense to have groups of one?
             $outputVariablesGroup = $this->getOutputVariablesGroups();
@@ -40,7 +41,9 @@ class TeamPerformancesToTeamPerformanceDifferencesLayer extends TrueSkillFactorG
     private function createTeamPerformanceToDifferenceFactor(
         Variable &$strongerTeam, Variable &$weakerTeam, Variable &$output)
     {
-        return new GaussianWeightedSumFactor($output, array($strongerTeam, $weakerTeam), array(1.0, -1.0));
+        $teams = array($strongerTeam, $weakerTeam);
+        $weights = array(1.0, -1.0);
+        return new GaussianWeightedSumFactor($output, $teams, $weights);
     }
 
     private function createOutputVariable()