diff --git a/.deepsource.toml b/.deepsource.toml
new file mode 100644
index 0000000..7f6697b
--- /dev/null
+++ b/.deepsource.toml
@@ -0,0 +1,9 @@
+version = 1
+
+[[analyzers]]
+name = "ruby"
+enabled = true
+
+[[analyzers]]
+name = "shell"
+enabled = true
\ No newline at end of file
diff --git a/lib/scientist/experiment.rb b/lib/scientist/experiment.rb
index f48b339..3a2e7ae 100644
--- a/lib/scientist/experiment.rb
+++ b/lib/scientist/experiment.rb
@@ -28,7 +28,7 @@ def self.set_default(klass)
   end
 
   # A mismatch, raised when raise_on_mismatches is enabled.
-  class MismatchError < Exception
+  class MismatchError < RuntimeError
     attr_reader :name, :result
 
     def initialize(name, result)
@@ -130,7 +130,7 @@ def clean_value(value)
   # and return true or false.
   #
   # Returns the block.
-  def compare(*args, &block)
+  def compare(*_args, &block)
     @_scientist_comparator = block
   end
 
@@ -140,7 +140,7 @@ def compare(*args, &block)
   # and return true or false.
   #
   # Returns the block.
-  def compare_errors(*args, &block)
+  def compare_errors(*_args, &block)
     @_scientist_error_comparator = block
   end
 
@@ -202,7 +202,7 @@ def raise_with(exception)
   # Called when an exception is raised while running an internal operation,
   # like :publish. Override this method to track these exceptions. The
   # default implementation re-raises the exception.
-  def raised(operation, error)
+  def raised(_operation, error)
     raise error
   end
 
@@ -290,6 +290,13 @@ def use(&block)
     try "control", &block
   end
 
+  # Define a block which will determine the cohort of this experiment
+  # when called. The block will be passed a `Scientist::Result` as its
+  # only argument and the cohort will be set on the result.
+  def cohort(&block)
+    @_scientist_determine_cohort = block
+  end
+
   # Whether or not to raise a mismatch error when a mismatch occurs.
   def raise_on_mismatches?
     if raise_on_mismatches.nil?
@@ -316,7 +323,7 @@ def generate_result(name)
     end
 
     control = observations.detect { |o| o.name == name }
-    Scientist::Result.new(self, observations, control)
+    Scientist::Result.new(self, observations, control, @_scientist_determine_cohort)
   end
 
   private
diff --git a/lib/scientist/result.rb b/lib/scientist/result.rb
index 76a4d21..79dc43b 100644
--- a/lib/scientist/result.rb
+++ b/lib/scientist/result.rb
@@ -19,19 +19,33 @@ class Scientist::Result
   # An Array of Observations in execution order.
   attr_reader :observations
 
+  # If the experiment was defined with a cohort block, the cohort this
+  # result has been determined to belong to.
+  attr_reader :cohort
+
   # Internal: Create a new result.
   #
-  # experiment    - the Experiment this result is for
-  # observations: - an Array of Observations, in execution order
-  # control:      - the control Observation
+  # experiment       - the Experiment this result is for
+  # observations:    - an Array of Observations, in execution order
+  # control:         - the control Observation
+  # determine_cohort - An optional callable that is passed the Result to
+  #                    determine its cohort
   #
-  def initialize(experiment, observations = [], control = nil)
+  def initialize(experiment, observations = [], control = nil, determine_cohort = nil)
     @experiment   = experiment
     @observations = observations
     @control      = control
     @candidates   = observations - [control]
     evaluate_candidates
 
+    if determine_cohort
+      begin
+        @cohort = determine_cohort.call(self)
+      rescue StandardError => e
+        experiment.raised :cohort, e
+      end
+    end
+
     freeze
   end
 
diff --git a/test/scientist/experiment_test.rb b/test/scientist/experiment_test.rb
index 12462f6..de859c1 100644
--- a/test/scientist/experiment_test.rb
+++ b/test/scientist/experiment_test.rb
@@ -149,7 +149,7 @@ def ex.enabled?
       true
     end
 
-    def ex.publish(result)
+    def ex.publish(_result)
       raise "boomtown"
     end
 
@@ -164,7 +164,7 @@ def ex.publish(result)
   end
 
   it "reports publishing errors" do
-    def @ex.publish(result)
+    def @ex.publish(_result)
       raise "boomtown"
     end
 
@@ -288,7 +288,7 @@ def @ex.enabled?
   end
 
   it "reports an error and returns the original value when an error is raised in a clean block" do
-    @ex.clean { |value| raise "kaboom" }
+    @ex.clean { |_value| raise "kaboom" }
 
     @ex.use { "control" }
     @ex.try { "candidate" }
@@ -302,6 +302,46 @@ def @ex.enabled?
     assert_equal "kaboom", exception.message
   end
 
+  describe "cohorts" do
+    it "accepts a cohort config block" do
+      @ex.cohort { "1" }
+    end
+
+    it "assigns a cohort to the result using the provided block" do
+      @ex.context(foo: "bar")
+      @ex.cohort { |res| "foo-#{res.context[:foo]}-#{Math.log10(res.control.value).round}" }
+      @ex.use { 5670 }
+      @ex.try { 5670 }
+
+      @ex.run
+      assert_equal "foo-bar-4", @ex.published_result.cohort
+    end
+
+    it "assigns no cohort if no cohort block passed" do
+      @ex.use { 5670 }
+      @ex.try { 5670 }
+
+      @ex.run
+      assert_nil @ex.published_result.cohort
+    end
+
+    it "rescues errors raised in the cohort determination block" do
+      @ex.use { 5670 }
+      @ex.try { 5670 }
+      @ex.cohort { |_res| raise "intentional" }
+
+      @ex.run
+
+      refute_nil @ex.published_result
+      assert_nil @ex.published_result.cohort
+
+      assert_equal 1, @ex.exceptions.size
+      code, exception = @ex.exceptions[0]
+      assert_equal :cohort, code
+      assert_equal "intentional", exception.message
+    end
+  end
+
   describe "#raise_with" do
     it "raises custom error if provided" do
       CustomError = Class.new(Scientist::Experiment::MismatchError)
@@ -372,9 +412,9 @@ def @ex.enabled?
 
     it "calls multiple ignore blocks to see if any match" do
       called_one = called_two = called_three = false
-      @ex.ignore { |a, b| called_one   = true; false }
-      @ex.ignore { |a, b| called_two   = true; false }
-      @ex.ignore { |a, b| called_three = true; false }
+      @ex.ignore { |_a, _b| called_one   = true; false }
+      @ex.ignore { |_a, _b| called_two   = true; false }
+      @ex.ignore { |_a, _b| called_three = true; false }
       refute @ex.ignore_mismatched_observation?(@a, @b)
       assert called_one
       assert called_two
@@ -383,9 +423,9 @@ def @ex.enabled?
 
     it "only calls ignore blocks until one matches" do
       called_one = called_two = called_three = false
-      @ex.ignore { |a, b| called_one   = true; false }
-      @ex.ignore { |a, b| called_two   = true; true  }
-      @ex.ignore { |a, b| called_three = true; false }
+      @ex.ignore { |_a, _b| called_one   = true; false }
+      @ex.ignore { |_a, _b| called_two   = true; true  }
+      @ex.ignore { |_a, _b| called_three = true; false }
       assert @ex.ignore_mismatched_observation?(@a, @b)
       assert called_one
       assert called_two
@@ -452,7 +492,7 @@ def @ex.raised(op, exception)
       @ex.clean { "So Clean" }
 
       err = assert_raises(Scientist::Experiment::MismatchError) { @ex.run }
-      assert_match /So Clean/, err.message
+      assert_match(/So Clean/, err.message)
     end
 
     it "doesn't raise when there is a mismatch if raise on mismatches is disabled" do
diff --git a/test/scientist/observation_test.rb b/test/scientist/observation_test.rb
index b788663..e58105c 100644
--- a/test/scientist/observation_test.rb
+++ b/test/scientist/observation_test.rb
@@ -145,7 +145,7 @@
     end
 
     it "doesn't clean nil values" do
-      @experiment.clean { |value| "foo" }
+      @experiment.clean { |_value| "foo" }
       a = Scientist::Observation.new("test", @experiment) { nil }
       assert_nil a.cleaned_value
     end
diff --git a/test/scientist/result_test.rb b/test/scientist/result_test.rb
index c9bc41e..9982718 100644
--- a/test/scientist/result_test.rb
+++ b/test/scientist/result_test.rb
@@ -80,7 +80,7 @@
     y = Scientist::Observation.new("y", @experiment) { :y }
     z = Scientist::Observation.new("z", @experiment) { :z }
 
-    @experiment.ignore { |control, candidate| candidate == :y }
+    @experiment.ignore { |_control, candidate| candidate == :y }
 
     result = Scientist::Result.new @experiment, [x, y, z], x
 
@@ -98,6 +98,17 @@
     assert_equal @experiment.name, result.experiment_name
   end
 
+  it "takes an optional callable to determine cohort" do
+    a = Scientist::Observation.new("a", @experiment) { 1 }
+    b = Scientist::Observation.new("b", @experiment) { 1 }
+
+    result = Scientist::Result.new @experiment, [a, b], a
+    assert_nil result.cohort
+
+    result = Scientist::Result.new @experiment, [a, b], a, ->(_res) { "cohort-1" }
+    assert_equal "cohort-1", result.cohort
+  end
+
   it "has the context from an experiment" do
     @experiment.context :foo => :bar
     a = Scientist::Observation.new("a", @experiment) { 1 }
diff --git a/test/scientist_test.rb b/test/scientist_test.rb
index 5e27026..c784650 100644
--- a/test/scientist_test.rb
+++ b/test/scientist_test.rb
@@ -28,7 +28,7 @@
     obj = Object.new
     obj.extend(Scientist)
 
-    assert_equal Hash.new, obj.default_scientist_context
+    assert_equal({}, obj.default_scientist_context)
   end
 
   it "respects default_scientist_context" do