Class: Vanity::Experiment::AbTest

Inherits:
Base show all
Defined in:
lib/vanity/experiment/ab_test.rb

Overview

The meat.

Instance Attribute Summary

Attributes inherited from Base

#completed_at, #id, #name

Class Method Summary collapse

Instance Method Summary collapse

Methods inherited from Base

#active?, #complete_if, #created_at, #description, #identify, load, #type, type

Constructor Details

#initialize(*args) ⇒ AbTest

Returns a new instance of AbTest.



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# File 'lib/vanity/experiment/ab_test.rb', line 94

def initialize(*args)
  super
end

Class Method Details

.friendly_name ⇒ Object 



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# File 'lib/vanity/experiment/ab_test.rb', line 88

def friendly_name
  "A/B Test" 
end

.probability(score) ⇒ Object

Convert z-score to probability.



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# File 'lib/vanity/experiment/ab_test.rb', line 82

def probability(score)
  score = score.abs
  probability = AbTest::Z_TO_PROBABILITY.find { |z,p| score >= z }
  probability ? probability.last : 0
end

Instance Method Details

#alternative(value) ⇒ Object

Returns an Alternative with the specified value.

Examples:

alternative(:red) == alternatives[0]
alternative(:blue) == alternatives[2]


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# File 'lib/vanity/experiment/ab_test.rb', line 154

def alternative(value)
  alternatives.find { |alt| alt.value == value }
end

#alternatives(*args) ⇒ Object

Call this method once to set alternative values for this experiment (requires at least two values). Call without arguments to obtain current list of alternatives.

Examples:

Define A/B test with three alternatives

ab_test "Background color" do
  metrics :coolness
  alternatives "red", "blue", "orange"
end

Find out which alternatives this test uses

alts = experiment(:background_color).alternatives
puts "#{alts.count} alternatives, with the colors: #{alts.map(&:value).join(", ")}"


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# File 'lib/vanity/experiment/ab_test.rb', line 131

def alternatives(*args)
  @alternatives = args.empty? ? [true, false] : args.clone
  class << self
    define_method :alternatives, instance_method(:_alternatives)
  end
  nil
end

#choose ⇒ Object

Chooses a value for this experiment. You probably want to use the Rails helper method ab_test instead.

This method picks an alternative for the current identity and returns the alternative’s value. It will consistently choose the same alternative for the same identity, and randomly split alternatives between different identities.

Examples:

color = experiment(:which_blue).choose


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# File 'lib/vanity/experiment/ab_test.rb', line 183

def choose
  if @playground.collecting?
    if active?
      identity = identity()
      index = connection.ab_showing(@id, identity)
      unless index
        index = alternative_for(identity)
        connection.ab_add_participant @id, index, identity
        check_completion!
      end
    else
      index = connection.ab_get_outcome(@id) || alternative_for(identity)
    end
  else
    identity = identity()
    @showing ||= {}
    @showing[identity] ||= alternative_for(identity)
  end
  @alternatives[index.to_i]
end

#chooses(value) ⇒ Object

Forces this experiment to use a particular alternative. You’ll want to use this from your test cases to test for the different alternatives.

Examples:

Setup test to red button

setup do
  experiment(:button_color).select(:red)
end

def test_shows_red_button
  . . .
end

Use nil to clear selection

teardown do
  experiment(:green_button).select(nil)
end


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# File 'lib/vanity/experiment/ab_test.rb', line 230

def chooses(value)
  if @playground.collecting?
    if value.nil?
      connection.ab_not_showing @id, identity
    else
      index = @alternatives.index(value)
      raise ArgumentError, "No alternative #{value.inspect} for #{name}" unless index
      connection.ab_show @id, identity, index
    end
  else
    @showing ||= {}
    @showing[identity] = value.nil? ? nil : @alternatives.index(value)
  end
  self
end

#complete! ⇒ Object 



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# File 'lib/vanity/experiment/ab_test.rb', line 376

def complete!
  return unless @playground.collecting? && active?
  super
  if @outcome_is
    begin
      result = @outcome_is.call
      outcome = result.id if result && result.experiment == self
    rescue
      # TODO: logging
    end
  else
    best = score.best
    outcome = best.id if best
  end
  # TODO: logging
  connection.ab_set_outcome @id, outcome || 0
end

#conclusion(score = score) ⇒ Object

Use the result of #score to derive a conclusion. Returns an array of claims.



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# File 'lib/vanity/experiment/ab_test.rb', line 306

def conclusion(score = score)
  claims = []
  participants = score.alts.inject(0) { |t,alt| t + alt.participants }
  claims << case participants
    when 0 ; "There are no participants in this experiment yet."
    when 1 ; "There is one participant in this experiment."
    else ; "There are #{participants} participants in this experiment."
  end
  # only interested in sorted alternatives with conversion
  sorted = score.alts.select { |alt| alt.measure > 0.0 }.sort_by(&:measure).reverse
  if sorted.size > 1
    # start with alternatives that have conversion, from best to worst,
    # then alternatives with no conversion.
    sorted |= score.alts
    # we want a result that's clearly better than 2nd best.
    best, second = sorted[0], sorted[1]
    if best.measure > second.measure
      diff = ((best.measure - second.measure) / second.measure * 100).round
      better = " (%d%% better than %s)" % [diff, second.name] if diff > 0
      claims << "The best choice is %s: it converted at %.1f%%%s." % [best.name, best.measure * 100, better]
      if best.probability >= 90
        claims << "With %d%% probability this result is statistically significant." % score.best.probability
      else
        claims << "This result is not statistically significant, suggest you continue this experiment."
      end
      sorted.delete best
    end
    sorted.each do |alt|
      if alt.measure > 0.0
        claims << "%s converted at %.1f%%." % [alt.name.gsub(/^o/, "O"), alt.measure * 100]
      else
        claims << "%s did not convert." % alt.name.gsub(/^o/, "O")
      end
    end
  else
    claims << "This experiment did not run long enough to find a clear winner."
  end
  claims << "#{score.choice.name.gsub(/^o/, "O")} selected as the best alternative." if score.choice
  claims
end

#destroy ⇒ Object

– Store/validate –



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# File 'lib/vanity/experiment/ab_test.rb', line 397

def destroy
  connection.destroy_experiment @id
  super
end

#false_true ⇒ Object Also known as: true_false

Defines an A/B test with two alternatives: false and true. This is the default pair of alternatives, so just syntactic sugar for those who love being explicit.

Examples:

ab_test "More bacon" do
  metrics :yummyness 
  false_true
end


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# File 'lib/vanity/experiment/ab_test.rb', line 168

def false_true
  alternatives false, true
end

#fingerprint(alternative) ⇒ Object

Returns fingerprint (hash) for given alternative. Can be used to lookup alternative for experiment without revealing what values are available (e.g. choosing alternative from HTTP query parameter).



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# File 'lib/vanity/experiment/ab_test.rb', line 207

def fingerprint(alternative)
  Digest::MD5.hexdigest("#{id}:#{alternative.id}")[-10,10]
end

#metrics(*args) ⇒ Object

Tells A/B test which metric we’re measuring, or returns metric in use.

Examples:

Define A/B test against coolness metric

ab_test "Background color" do
  metrics :coolness
  alternatives "red", "blue", "orange"
end

Find metric for A/B test

puts "Measures: " + experiment(:background_color).metrics.map(&:name)


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# File 'lib/vanity/experiment/ab_test.rb', line 110

def metrics(*args)
  @metrics = args.map { |id| @playground.metric(id) } unless args.empty?
  @metrics
end

#outcome ⇒ Object

Alternative chosen when this experiment completed.



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# File 'lib/vanity/experiment/ab_test.rb', line 370

def outcome
  return unless @playground.collecting?
  outcome = connection.ab_get_outcome(@id)
  outcome && _alternatives[outcome]
end

#outcome_is(&block) ⇒ Object

Defines how the experiment can choose the optimal outcome on completion.

By default, Vanity will take the best alternative (highest conversion rate) and use that as the outcome. You experiment may have different needs, maybe you want the least performing alternative, or factor cost in the equation?

The default implementation reads like this:

outcome_is do
  a, b = alternatives
  # a is expensive, only choose a if it performs 2x better than b
  a.measure > b.measure * 2 ? a : b
end

Raises:

  • (ArgumentError) 


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# File 'lib/vanity/experiment/ab_test.rb', line 363

def outcome_is(&block)
  raise ArgumentError, "Missing block" unless block
  raise "outcome_is already called on this experiment" if @outcome_is
  @outcome_is = block
end

#save ⇒ Object 



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# File 'lib/vanity/experiment/ab_test.rb', line 402

def save
  true_false unless @alternatives
  fail "Experiment #{name} needs at least two alternatives" unless @alternatives.size >= 2
  super
  if @metrics.nil? || @metrics.empty?
    warn "Please use metrics method to explicitly state which metric you are measuring against."
    metric = @playground.metrics[id] ||= Vanity::Metric.new(@playground, name)
    @metrics = [metric]
  end
  @metrics.each do |metric|
    metric.hook &method(:track!)
  end
end

#score(probability = 90) ⇒ Object

Scores alternatives based on the current tracking data. This method returns a structure with the following attributes:

:alts

Ordered list of alternatives, populated with scoring info.

:base

Second best performing alternative.

:least

Least performing alternative (but more than zero conversion).

:choice

Choice alterntive, either the outcome or best alternative.

Alternatives returned by this method are populated with the following attributes:

:z_score

Z-score (relative to the base alternative).

:probability

Probability (z-score mapped to 0, 90, 95, 99 or 99.9%).

:difference

Difference from the least performant altenative.

The choice alternative is set only if its probability is higher or equal to the specified probability (default is 90%).



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# File 'lib/vanity/experiment/ab_test.rb', line 275

def score(probability = 90)
  alts = alternatives
  # sort by conversion rate to find second best and 2nd best
  sorted = alts.sort_by(&:measure)
  base = sorted[-2]
  # calculate z-score
  pc = base.measure
  nc = base.participants
  alts.each do |alt|
    p = alt.measure
    n = alt.participants
    alt.z_score = (p - pc) / ((p * (1-p)/n) + (pc * (1-pc)/nc)).abs ** 0.5
    alt.probability = AbTest.probability(alt.z_score)
  end
  # difference is measured from least performant
  if least = sorted.find { |alt| alt.measure > 0 }
    alts.each do |alt|
      if alt.measure > least.measure
        alt.difference = (alt.measure - least.measure) / least.measure * 100
      end
    end
  end
  # best alternative is one with highest conversion rate (best shot).
  # choice alternative can only pick best if we have high probability (>90%).
  best = sorted.last if sorted.last.measure > 0.0
  choice = outcome ? alts[outcome.id] : (best && best.probability >= probability ? best : nil)
  Struct.new(:alts, :best, :base, :least, :choice).new(alts, best, base, least, choice)
end

#showing?(alternative) ⇒ Boolean

True if this alternative is currently showing (see #chooses).

Returns:

  • (Boolean) 


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# File 'lib/vanity/experiment/ab_test.rb', line 247

def showing?(alternative)
  identity = identity()
  if @playground.collecting?
    connection.ab_showing(@id, identity) == alternative.id
  else
    @showing ||= {}
    @showing[identity] == alternative.id
  end
end

#track!(metric_id, timestamp, count, *args) ⇒ Object

Called when tracking associated metric.



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# File 'lib/vanity/experiment/ab_test.rb', line 417

def track!(metric_id, timestamp, count, *args)
  return unless active?
  identity = identity() rescue nil
  if identity
    return if connection.ab_showing(@id, identity)
    index = alternative_for(identity)
    connection.ab_add_conversion @id, index, identity, count
    check_completion!
  end
end