diff --git a/src/FactorGraphs/DefaultVariable.php b/src/FactorGraphs/DefaultVariable.php
deleted file mode 100644
index 96cc5f0..0000000
--- a/src/FactorGraphs/DefaultVariable.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-namespace DNW\Skills\FactorGraphs;
-
-use Exception;
-
-// XXX: This class is not used anywhere
-class DefaultVariable extends Variable
-{
-    public function __construct()
-    {
-        parent::__construct('Default', null);
-    }
-
-    public function getValue()
-    {
-        return null;
-    }
-
-    public function setValue($value): never
-    {
-        throw new Exception();
-    }
-}
diff --git a/src/FactorGraphs/Factor.php b/src/FactorGraphs/Factor.php
index 1440a12..2a3fb66 100644
--- a/src/FactorGraphs/Factor.php
+++ b/src/FactorGraphs/Factor.php
@@ -102,7 +102,7 @@ abstract class Factor implements \Stringable
 
     abstract public function createVariableToMessageBinding(Variable $variable);
 
-    protected function createVariableToMessageBindingWithMessage(Variable $variable, Message $message)
+    protected function createVariableToMessageBindingWithMessage(Variable $variable, Message $message): Message
     {
         $this->_messageToVariableBinding->setValue($message, $variable);
         $this->_messages[] = $message;
diff --git a/src/FactorGraphs/KeyedVariable.php b/src/FactorGraphs/KeyedVariable.php
index 465e10b..f064f0c 100644
--- a/src/FactorGraphs/KeyedVariable.php
+++ b/src/FactorGraphs/KeyedVariable.php
@@ -4,12 +4,12 @@ namespace DNW\Skills\FactorGraphs;
 
 class KeyedVariable extends Variable
 {
-    public function __construct(private $_key, $name, $prior)
+    public function __construct(private mixed $_key, string $name, mixed $prior)
     {
         parent::__construct($name, $prior);
     }
 
-    public function getKey()
+    public function getKey(): mixed
     {
         return $this->_key;
     }
diff --git a/src/FactorGraphs/Variable.php b/src/FactorGraphs/Variable.php
index f3e8f42..81a2139 100644
--- a/src/FactorGraphs/Variable.php
+++ b/src/FactorGraphs/Variable.php
@@ -24,7 +24,7 @@ class Variable implements \Stringable
         $this->_value = $value;
     }
 
-    public function resetToPrior()
+    public function resetToPrior(): void
     {
         $this->_value = $this->_prior;
     }
diff --git a/src/FactorGraphs/VariableFactory.php b/src/FactorGraphs/VariableFactory.php
index c20c73f..d645c0b 100644
--- a/src/FactorGraphs/VariableFactory.php
+++ b/src/FactorGraphs/VariableFactory.php
@@ -8,14 +8,14 @@ class VariableFactory
     {
     }
 
-    public function createBasicVariable($name)
+    public function createBasicVariable(string $name): Variable
     {
         $initializer = $this->_variablePriorInitializer;
 
         return new Variable($name, $initializer());
     }
 
-    public function createKeyedVariable($key, $name)
+    public function createKeyedVariable(mixed $key, string $name): KeyedVariable
     {
         $initializer = $this->_variablePriorInitializer;
 
diff --git a/src/HashMap.php b/src/HashMap.php
index b45056c..b951eb9 100644
--- a/src/HashMap.php
+++ b/src/HashMap.php
@@ -11,14 +11,14 @@ class HashMap
 
     private array $_hashToKey = [];
 
-    public function getValue($key)
+    public function getValue(string|object $key): mixed
     {
         $hash = self::getHash($key);
 
         return $this->_hashToValue[$hash];
     }
 
-    public function setValue($key, $value)
+    public function setValue(string|object $key, mixed $value): self
     {
         $hash = self::getHash($key);
         $this->_hashToKey[$hash] = $key;
@@ -27,22 +27,22 @@ class HashMap
         return $this;
     }
 
-    public function getAllKeys()
+    public function getAllKeys(): array
     {
         return array_values($this->_hashToKey);
     }
 
-    public function getAllValues()
+    public function getAllValues(): array
     {
         return array_values($this->_hashToValue);
     }
 
-    public function count()
+    public function count(): int
     {
         return count($this->_hashToKey);
     }
 
-    private static function getHash($key)
+    private static function getHash(string|Object $key): string
     {
         if (is_object($key)) {
             return spl_object_hash($key);
diff --git a/src/RatingContainer.php b/src/RatingContainer.php
index ec62d4c..98532a7 100644
--- a/src/RatingContainer.php
+++ b/src/RatingContainer.php
@@ -4,34 +4,34 @@ namespace DNW\Skills;
 
 class RatingContainer
 {
-    private $_playerToRating;
+    private HashMap $_playerToRating;
 
     public function __construct()
     {
         $this->_playerToRating = new HashMap();
     }
 
-    public function getRating(Player $player)
+    public function getRating(Player $player): mixed
     {
         return $this->_playerToRating->getValue($player);
     }
 
-    public function setRating(Player $player, Rating $rating)
+    public function setRating(Player $player, Rating $rating): HashMap
     {
         return $this->_playerToRating->setValue($player, $rating);
     }
 
-    public function getAllPlayers()
+    public function getAllPlayers(): array
     {
         return $this->_playerToRating->getAllKeys();
     }
 
-    public function getAllRatings()
+    public function getAllRatings(): array
     {
         return $this->_playerToRating->getAllValues();
     }
 
-    public function count()
+    public function count(): int
     {
         return $this->_playerToRating->count();
     }
diff --git a/src/TrueSkill/DrawMargin.php b/src/TrueSkill/DrawMargin.php
index 83d1353..9f09d2c 100644
--- a/src/TrueSkill/DrawMargin.php
+++ b/src/TrueSkill/DrawMargin.php
@@ -6,7 +6,7 @@ use DNW\Skills\Numerics\GaussianDistribution;
 
 final class DrawMargin
 {
-    public static function getDrawMarginFromDrawProbability($drawProbability, $beta)
+    public static function getDrawMarginFromDrawProbability(float $drawProbability, float $beta): float
     {
         // Derived from TrueSkill technical report (MSR-TR-2006-80), page 6
 
diff --git a/src/TrueSkill/Factors/GaussianFactor.php b/src/TrueSkill/Factors/GaussianFactor.php
index 82dd312..98ed2a0 100644
--- a/src/TrueSkill/Factors/GaussianFactor.php
+++ b/src/TrueSkill/Factors/GaussianFactor.php
@@ -22,7 +22,7 @@ abstract class GaussianFactor extends Factor
         return $logZ;
     }
 
-    public function createVariableToMessageBinding(Variable $variable)
+    public function createVariableToMessageBinding(Variable $variable): Message
     {
         $newDistribution = GaussianDistribution::fromPrecisionMean(0, 0);