diff --git a/benchmark/BasicBench.php b/benchmark/BasicBench.php
index 29b17fa..2303fc9 100644
--- a/benchmark/BasicBench.php
+++ b/benchmark/BasicBench.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace DNW\Skills\Benchmark;
 
 use DNW\Skills\TrueSkill\TwoPlayerTrueSkillCalculator;
diff --git a/tests/GuardTest.php b/tests/GuardTest.php
index 44213ef..18d95af 100644
--- a/tests/GuardTest.php
+++ b/tests/GuardTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests;
 
diff --git a/tests/Numerics/MatrixTest.php b/tests/Numerics/MatrixTest.php
index ce1b651..70843a1 100644
--- a/tests/Numerics/MatrixTest.php
+++ b/tests/Numerics/MatrixTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\Numerics;
 
diff --git a/tests/Numerics/RangeTest.php b/tests/Numerics/RangeTest.php
index 7b44bb8..b5504b4 100644
--- a/tests/Numerics/RangeTest.php
+++ b/tests/Numerics/RangeTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\Numerics;
 
diff --git a/tests/PlayerTest.php b/tests/PlayerTest.php
index de7176f..9d14b37 100644
--- a/tests/PlayerTest.php
+++ b/tests/PlayerTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests;
 
diff --git a/tests/TrueSkill/DrawMarginTest.php b/tests/TrueSkill/DrawMarginTest.php
index 3c37fe1..099a995 100644
--- a/tests/TrueSkill/DrawMarginTest.php
+++ b/tests/TrueSkill/DrawMarginTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\TrueSkill;
 
diff --git a/tests/TrueSkill/FactorGraphTeamTrueSkillCalculatorTest.php b/tests/TrueSkill/FactorGraphTeamTrueSkillCalculatorTest.php
index a7a7ce6..8ca133d 100644
--- a/tests/TrueSkill/FactorGraphTeamTrueSkillCalculatorTest.php
+++ b/tests/TrueSkill/FactorGraphTeamTrueSkillCalculatorTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\TrueSkill;
 
diff --git a/tests/TrueSkill/FactorGraphTrueSkillCalculatorTest.php b/tests/TrueSkill/FactorGraphTrueSkillCalculatorTest.php
index 70c9d9d..8d07888 100644
--- a/tests/TrueSkill/FactorGraphTrueSkillCalculatorTest.php
+++ b/tests/TrueSkill/FactorGraphTrueSkillCalculatorTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\TrueSkill;
 
diff --git a/tests/TrueSkill/TrueSkillCalculatorTests.php b/tests/TrueSkill/TrueSkillCalculatorTests.php
index d606b9c..4ef743e 100644
--- a/tests/TrueSkill/TrueSkillCalculatorTests.php
+++ b/tests/TrueSkill/TrueSkillCalculatorTests.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\TrueSkill;
 
diff --git a/tests/TrueSkill/TwoPlayerTrueSkillCalculatorTest.php b/tests/TrueSkill/TwoPlayerTrueSkillCalculatorTest.php
index e99243f..97b8466 100644
--- a/tests/TrueSkill/TwoPlayerTrueSkillCalculatorTest.php
+++ b/tests/TrueSkill/TwoPlayerTrueSkillCalculatorTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\TrueSkill;
 
diff --git a/tests/TrueSkill/TwoTeamTrueSkillCalculatorTest.php b/tests/TrueSkill/TwoTeamTrueSkillCalculatorTest.php
index 533c238..dd1edf2 100644
--- a/tests/TrueSkill/TwoTeamTrueSkillCalculatorTest.php
+++ b/tests/TrueSkill/TwoTeamTrueSkillCalculatorTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace DNW\Skills\Tests\TrueSkill;