diff --git a/build.xml b/build.xml
index fa38d16..ebb8ffc 100644
--- a/build.xml
+++ b/build.xml
@@ -3,7 +3,7 @@
     <!-- Execute code style check. -->
     <target name="phpcs">
         <echo>Running PHP PSR-2 code style checks.</echo>
-        <exec executable="vendor/bin/phpcs" passthru="true">
+        <exec executable="vendor/bin/phpcs" passthru="true" checkreturn="true">
             <arg value="--colors" />
             <arg value="--standard=PSR2" />
             <arg value="src" />
@@ -23,7 +23,7 @@
     <!-- Execute PHP linting. -->
     <target name="phplint">
         <echo>Running PHP parallel linter.</echo>
-        <exec executable="vendor/bin/parallel-lint" passthru="true">
+        <exec executable="vendor/bin/parallel-lint" passthru="true" checkreturn="true">
             <arg value="src" />
         </exec>
     </target>
@@ -31,7 +31,7 @@
     <!-- Execute PHPUnit tests. -->
     <target name="phpunit">
         <echo>Running PHP Unit tests.</echo>
-        <exec executable="vendor/bin/phpunit" passthru="true">
+        <exec executable="vendor/bin/phpunit" passthru="true" checkreturn="true">
             <arg value="--color" />
         </exec>
     </target>
diff --git a/composer.json b/composer.json
index e3036b7..0bdc7e2 100644
--- a/composer.json
+++ b/composer.json
@@ -11,7 +11,8 @@
         }
     ],
     "require": {
-        "php": "^5.5 || ^7.0"
+        "php": "^5.5 || ^7.0",
+        "ircmaxell/random-lib": "^1.2"
     },
     "require-dev": {
         "phpunit/phpunit": ">=4.8",
diff --git a/src/Chances/Chance.php b/src/Chances/Chance.php
new file mode 100644
index 0000000..0af2cc0
--- /dev/null
+++ b/src/Chances/Chance.php
@@ -0,0 +1,7 @@
+<?php
+namespace Scientist\Chances;
+
+interface Chance
+{
+    public function shouldRun();
+}
diff --git a/src/Chances/StandardChance.php b/src/Chances/StandardChance.php
new file mode 100644
index 0000000..271d400
--- /dev/null
+++ b/src/Chances/StandardChance.php
@@ -0,0 +1,57 @@
+<?php
+namespace Scientist\Chances;
+
+use RandomLib\Factory;
+use RandomLib\Generator;
+
+class StandardChance implements Chance
+{
+    private $generator;
+
+    private $percentage = 100;
+
+    /**
+     * StandardChance constructor.
+     * @param Generator|null $generator
+     */
+    public function __construct(Generator $generator = null)
+    {
+        if ($generator === null) {
+            $factory = new Factory;
+            $generator = $factory->getLowStrengthGenerator();
+        }
+        $this->generator = $generator;
+    }
+
+    /**
+     * Determine whether or not the experiment should run
+     */
+    public function shouldRun()
+    {
+        if ($this->percentage == 0) {
+            return false;
+        }
+
+        $random = $this->generator
+            ->generateInt(0, 100);
+        return $random <= $this->percentage;
+    }
+
+    /**
+     * @return int
+     */
+    public function getPercentage()
+    {
+        return $this->percentage;
+    }
+
+    /**
+     * @param int $percentage
+     * @return $this
+     */
+    public function setPercentage($percentage)
+    {
+        $this->percentage = $percentage;
+        return $this;
+    }
+}
diff --git a/src/Experiment.php b/src/Experiment.php
index d04fdf5..3ae831f 100644
--- a/src/Experiment.php
+++ b/src/Experiment.php
@@ -2,6 +2,8 @@
 
 namespace Scientist;
 
+use Scientist\Chances\Chance;
+use Scientist\Chances\StandardChance;
 use Scientist\Matchers\Matcher;
 use Scientist\Matchers\StandardMatcher;
 
@@ -60,9 +62,9 @@ class Experiment
     /**
      * Execution chance.
      *
-     * @var integer
+     * @var \Scientist\Chances\Chance
      */
-    protected $chance = 100;
+    protected $chance;
 
     /**
      * Create a new experiment.
@@ -75,6 +77,7 @@ public function __construct($name, Laboratory $laboratory)
         $this->name = $name;
         $this->laboratory = $laboratory;
         $this->matcher = new StandardMatcher;
+        $this->chance = new StandardChance;
     }
 
     /**
@@ -185,13 +188,13 @@ public function getMatcher()
     /**
      * Set the execution chance.
      *
-     * @param integer $chance
+     * @param Chances\Chance $chance
      *
      * @return $this
      */
-    public function chance($chance)
+    public function chance(Chance $chance)
     {
-        $this->chance = (int) $chance;
+        $this->chance = $chance;
 
         return $this;
     }
@@ -199,7 +202,7 @@ public function chance($chance)
     /**
      * Get the execution chance.
      *
-     * @return integer
+     * @return Chances\Chance
      */
     public function getChance()
     {
@@ -213,7 +216,8 @@ public function getChance()
      */
     public function shouldRun()
     {
-        return rand(0, 100) <= $this->chance;
+        return $this->chance
+            ->shouldRun();
     }
 
     /**
diff --git a/src/Machine.php b/src/Machine.php
index 8ddb522..0da891a 100644
--- a/src/Machine.php
+++ b/src/Machine.php
@@ -52,7 +52,6 @@ public function __construct(callable $callback, array $params = [], $muted = fal
         $this->params   = $params;
         $this->muted    = $muted;
         $this->result   = new Result;
-
     }
 
     /**
diff --git a/tests/Chances/StandardChanceTest.php b/tests/Chances/StandardChanceTest.php
new file mode 100644
index 0000000..4001849
--- /dev/null
+++ b/tests/Chances/StandardChanceTest.php
@@ -0,0 +1,156 @@
+<?php
+namespace Scientist\Chances;
+
+use RandomLib\Generator;
+
+class StandardChanceTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @var StandardChance
+     */
+    private $chance;
+
+    /**
+     * @var Generator
+     */
+    private $generator;
+
+    public function setUp()
+    {
+        $this->generator = $this->getMockGenerator();
+        $this->chance = new StandardChance($this->generator);
+    }
+
+    public function test_that_standard_chance_is_an_instance_of_chance()
+    {
+        $chance = new StandardChance();
+        $this->assertInstanceOf('\Scientist\Chances\Chance', $chance);
+    }
+
+    public function test_that_a_random_number_generator_is_created_upon_instantiation()
+    {
+        $chance = new StandardChance();
+        $reflection = new \ReflectionClass($chance);
+        $property = $reflection->getProperty('generator');
+        $property->setAccessible(true);
+
+        $this->assertInstanceOf('\RandomLib\Generator', $property->getValue($chance));
+    }
+
+    public function test_that_it_takes_a_custom_random_number_generator_in_the_constructor()
+    {
+        $reflection = new \ReflectionClass($this->chance);
+        $property = $reflection->getProperty('generator');
+        $property->setAccessible(true);
+
+        $this->assertSame($this->generator, $property->getValue($this->chance));
+    }
+
+    /**
+     * @dataProvider percentageDataProvider
+     */
+    public function test_that_should_run_returns_true_when_the_chance_is_100($random)
+    {
+        $this->generator
+            ->expects($this->once())
+            ->method('generateInt')
+            ->with(0, 100)
+            ->willReturn($random);
+
+        $this->assertTrue($this->chance->shouldRun());
+    }
+
+    public function test_that_the_default_percentage_is_100()
+    {
+        $this->assertEquals(100, $this->chance->getPercentage());
+    }
+
+    public function test_that_set_percentage_sets_the_percentage()
+    {
+        $percentage = rand(1, 100);
+        $this->chance
+            ->setPercentage($percentage);
+        $this->assertEquals($percentage, $this->chance->getPercentage());
+    }
+
+    public function test_that_set_percentage_returns_the_chance_object_for_chaining()
+    {
+        $percentage = rand(1, 100);
+        $this->assertSame($this->chance, $this->chance->setPercentage($percentage));
+    }
+
+    public function test_that_should_run_always_returns_false_when_percentage_is_zero()
+    {
+        $this->generator
+            ->expects($this->never())
+            ->method('generateInt');
+        $this->chance
+            ->setPercentage(0);
+        $this->assertFalse($this->chance->shouldRun());
+    }
+
+    /**
+     * @dataProvider nonZeroPercentageDataProvider
+     * @param integer $percentage Percentage of the time to run
+     */
+    public function test_that_it_returns_true_when_percentage_is_greater_than_the_generated_number($percentage)
+    {
+        $this->generator
+            ->expects($this->once())
+            ->method('generateInt')
+            ->with(0, 100)
+            ->willReturn($percentage - 1);
+
+        $this->chance
+            ->setPercentage($percentage);
+
+        $this->assertTrue($this->chance->shouldRun());
+    }
+
+    /**
+     * @dataProvider nonZeroPercentageDataProvider
+     * @param integer $percentage Percentage of the time to run
+     */
+    public function test_that_it_returns_false_when_percentage_is_less_than_the_generated_number($percentage)
+    {
+        $this->generator
+            ->expects($this->once())
+            ->method('generateInt')
+            ->with(0, 100)
+            ->willReturn($percentage + 1);
+
+        $this->chance
+            ->setPercentage($percentage);
+
+        $this->assertFalse($this->chance->shouldRun());
+    }
+
+    /**
+     * @return array
+     */
+    public function nonZeroPercentageDataProvider()
+    {
+        $percentages = $this->percentageDataProvider();
+        array_shift($percentages);
+        return $percentages;
+    }
+
+    /**
+     * Data provider to cover all 100 percentage values
+     * @return array
+     */
+    public function percentageDataProvider()
+    {
+        return array_map(function ($value) {
+            return [$value];
+        }, range(0, 100));
+    }
+
+    public function getMockGenerator()
+    {
+        return $this->getMockBuilder('\RandomLib\Generator')
+            ->disableOriginalConstructor()
+            ->disableProxyingToOriginalMethods()
+            ->getMock();
+    }
+}
diff --git a/tests/ExperimentTest.php b/tests/ExperimentTest.php
index 43e0220..eb5ce88 100644
--- a/tests/ExperimentTest.php
+++ b/tests/ExperimentTest.php
@@ -63,9 +63,10 @@ public function test_that_multiple_trial_callbacks_can_be_defined()
 
     public function test_that_a_chance_variable_can_be_set()
     {
+        $chance = $this->getMock('\Scientist\Chances\Chance');
         $e = new Experiment('test experiment', new Laboratory);
-        $e->chance(50);
-        $this->assertEquals(50, $e->getChance());
+        $e->chance($chance);
+        $this->assertEquals($chance, $e->getChance());
     }
 
     public function test_that_an_experiment_matcher_can_be_set()
@@ -93,12 +94,25 @@ public function test_that_running_experiment_with_no_laboratory_executes_control
 
     public function test_that_running_experiment_with_zero_chance_executes_control()
     {
+        $chance = $this->getMockChance();
+        $chance->expects($this->once())
+            ->method('shouldRun')
+            ->willReturn(false);
+
         $l = new Laboratory;
         $v = $l->experiment('test experiment')
             ->control(function () { return 'foo'; })
-            ->chance(0)
+            ->chance($chance)
             ->run();
 
         $this->assertEquals('foo', $v);
     }
+
+    public function getMockChance()
+    {
+        return $this->getMockBuilder('\Scientist\Chances\Chance')
+            ->disableOriginalConstructor()
+            ->disableProxyingToOriginalMethods()
+            ->getMock();
+    }
 }