Split is a rack based ab testing framework designed to work with Rails, Sinatra or any other rack based app.

Split is heavily inspired by the Abingo and Vanity rails ab testing plugins and Resque in its use of Redis.

Split is designed to be hacker friendly, allowing for maximum customisation and extensibility.

Gem Version Build Status Dependency Status Code Climate Coverage Status


Split uses redis as a datastore.

Split only supports redis 2.0 or greater.

If you're on OS X, Homebrew is the simplest way to install Redis:

$ brew install redis
$ redis-server /usr/local/etc/redis.conf

You now have a Redis daemon running on 6379.


If you are using bundler add split to your Gemfile:

gem 'split'

Then run:

$ bundle install

Otherwise install the gem:

$ gem install split

and require it in your project:

require 'split'


If you are using Redis on Ruby 1.8.x then you will likely want to also use the SystemTimer gem if you want to make sure the Redis client will not hang.

Put the following in your gemfile as well:

gem 'SystemTimer'

Rails 3

Split is autoloaded when rails starts up, as long as you've configured redis it will 'just work'.

Rails 2.3

To configure Rails 2.3 with Split you need to mix in the helper methods. Add the following lines to config/initializers.split.rb:

ActionController::Base.send :include, Split::Helper
ActionController::Base.helper Split::Helper


To configure sinatra with Split you need to enable sessions and mix in the helper methods. Add the following lines at the top of your sinatra app:

class MySinatraApp < Sinatra::Base
  enable :sessions
  helpers Split::Helper

  get '/' do


To begin your ab test use the ab_test method, naming your experiment with the first argument and then the different alternatives which you wish to test on as the other arguments.

ab_test returns one of the alternatives, if a user has already seen that test they will get the same alternative as before, which you can use to split your code on.

It can be used to render different templates, show different text or any other case based logic.

finished is used to make a completion of an experiment, or conversion.

Example: View

<% ab_test("login_button", "/images/button1.jpg", "/images/button2.jpg") do |button_file| %>
  <%= img_tag(button_file, :alt => "Login!") %>
<% end %>

Example: Controller

def register_new_user
  # See what level of free points maximizes users' decision to buy replacement points.
  @starter_points = ab_test("new_user_free_points", '100', '200', '300')

Example: Conversion tracking (in a controller!)

def buy_new_points
  # some business logic

Example: Conversion tracking (in a view)

Thanks for signing up, dude! <% finished("signup_page_redesign") %>

You can find more examples, tutorials and guides on the wiki.

Statistical Validity

Split uses a z test (n>30) of the difference between your control and alternative conversion rates to calculate statistical significance.

This means that Split will tell you whether an alternative is better or worse than your control, but it will not distinguish between which alternative is the best in an experiment with multiple alternatives. To find that out, run a new experiment with one of the prior alternatives as the control.

Also, as per this blog post on the pitfalls of A/B testing, it is highly recommended that you determine your requisite sample size for each branch before running the experiment. Otherwise, you'll have an increased rate of false positives (experiments which show a significant effect where really there is none).

Here is a sample size calculator for your convenience.

Finally, two things should be noted about the dashboard:

  • Split will only tell if you if your experiment is 90%, 95%, or 99% significant. For levels of lesser significance, Split will simply show "insufficient significance."
  • If you have less than 30 participants or 5 conversions for a branch, Split will not calculate significance, as you have not yet gathered enough data.


Weighted alternatives

Perhaps you only want to show an alternative to 10% of your visitors because it is very experimental or not yet fully load tested.

To do this you can pass a weight with each alternative in the following ways:

ab_test('homepage design', {'Old' => 20}, {'New' => 2})

ab_test('homepage design', 'Old', {'New' => 0.1})

ab_test('homepage design', {'Old' => 10}, 'New')

Note: If using ruby 1.8.x and weighted alternatives you should always pass the control alternative through as the second argument with any other alternatives as a third argument because the order of the hash is not preserved in ruby 1.8, ruby 1.9.1+ users are not affected by this bug.

This will only show the new alternative to visitors 1 in 10 times, the default weight for an alternative is 1.

Overriding alternatives

For development and testing, you may wish to force your app to always return an alternative. You can do this by passing it as a parameter in the url.

If you have an experiment called button_color with alternatives called red and blue used on your homepage, a url such as:


will always have red buttons. This won't be stored in your session or count towards to results, unless you set the store_override configuration option.

Starting experiments manually

By default new AB tests will be active right after deployment. In case you would like to start new test a while after the deploy, you can do it by setting the start_manually configuration option to true.

After choosing this option tests won't be started right after deploy, but after pressing the Start button in Split admin dashboard.

Reset after completion

When a user completes a test their session is reset so that they may start the test again in the future.

To stop this behaviour you can pass the following option to the finished method:

finished('experiment_name', :reset => false)

The user will then always see the alternative they started with.

Multiple experiments at once

By default Split will avoid users participating in multiple experiments at once. This means you are less likely to skew results by adding in more variation to your tests.

To stop this behaviour and allow users to participate in multiple experiments at once enable the allow_multiple_experiments config option like so:

Split.configure do |config|
  config.allow_multiple_experiments = true

Experiment Persistence

Split comes with three built-in persistence adapters for storing users and the alternatives they've been given for each experiment.

By default Split will store the tests for each user in the session.

You can optionally configure Split to use a cookie, Redis, or any custom adapter of your choosing.


Split.configure do |config|
  config.persistence = :cookie

Note: Using cookies depends on ActionDispatch::Cookies or any identical API


Using Redis will allow ab_users to persist across sessions or machines.

Split.configure do |config|
  config.persistence = Split::Persistence::RedisAdapter.with_config(:lookup_by => proc { |context| context.current_user_id }
  # Equivalent
  # config.persistence = Split::Persistence::RedisAdapter.with_config(:lookup_by => :current_user_id }


  • lookup_by: method to invoke per request for uniquely identifying ab_users (mandatory configuration)
  • namespace: separate namespace to store these persisted values (default "persistence")

Custom Adapter

Your custom adapter needs to implement the same API as existing adapters. See Split::Persistance::CookieAdapter or Split::Persistence::SessionAdapter for a starting point.

Split.configure do |config|
  config.persistence = YourCustomAdapterClass

Trial Event Hooks

You can define methods that will be called at the same time as experiment alternative participation and goal completion.

For example:

Split.configure do |config|
  config.on_trial_choose   = :log_trial_choice
  config.on_trial_complete = :log_trial_complete

Set these attributes to a method name available in the same context as the ab_test method. These methods should accept one argument, a Trial instance.

def log_trial_choose(trial)
  logger.info "experiment=%s alternative=%s user=%s" %
    [ trial.experiment.name, trial.alternative, current_user.id ]

def log_trial_complete(trial)
  logger.info "experiment=%s alternative=%s user=%s complete=true" %
    [ trial.experiment.name, trial.alternative, current_user.id ]


If you are running ab_test from a view, you must define your event hook callback as a helper_method in the controller:

helper_method :log_trial_choose

def log_trial_choose(trial)
  logger.info "experiment=%s alternative=%s user=%s" %
    [ trial.experiment.name, trial.alternative, current_user.id ]

Experiment Hooks

You can assign a proc that will be called when an experiment is reset or deleted. You can use these hooks to call methods within your application to keep data related to experiments in sync with Split.

For example:

Split.configure do |config|
  config.on_experiment_reset  = proc{ |experiment| # Do something on reset }
  config.on_experiment_delete = proc{ |experiment| # Do something else on delete }

Web Interface

Split comes with a Sinatra-based front end to get an overview of how your experiments are doing.

If you are running Rails 2: You can mount this inside your app using Rack::URLMap in your config.ru

require 'split/dashboard'

run Rack::URLMap.new \
  "/"       => Your::App.new,
  "/split" => Split::Dashboard.new

However, if you are using Rails 3: You can mount this inside your app routes by first adding this to the Gemfile:

gem 'split', :require => 'split/dashboard'

Then adding this to config/routes.rb

mount Split::Dashboard, :at => 'split'

You may want to password protect that page, you can do so with Rack::Auth::Basic (in your split initializer file)

Split::Dashboard.use Rack::Auth::Basic do |username, password|
  username == 'admin' && password == 'p4s5w0rd'

You can even use Devise or any other Warden-based authentication method to authorize users. Just replace mount Split::Dashboard, :at => 'split' in config/routes.rb with the following:

match "/split" => Split::Dashboard, :anchor => false, :constraints => lambda { |request|
  request.env['warden'].authenticated? # are we authenticated?
  request.env['warden'].authenticate! # authenticate if not already
  # or even check any other condition such as request.env['warden'].user.is_admin?

More information on this here




You can override the default configuration options of Split like so:

Split.configure do |config|
  config.db_failover = true # handle redis errors gracefully
  config.db_failover_on_db_error = proc{|error| Rails.logger.error(error.message) }
  config.allow_multiple_experiments = true
  config.enabled = true
  config.persistence = Split::Persistence::SessionAdapter
  #config.start_manually = false ## new test will have to be started manually from the admin panel. default false
  config.include_rails_helper = true


In most scenarios you don't want to have AB-Testing enabled for web spiders, robots or special groups of users. Split provides functionality to filter this based on a predefined, extensible list of bots, IP-lists or custom exclude logic.

Split.configure do |config|
  # bot config
  config.robot_regex = /my_custom_robot_regex/ # or
  config.bots['newbot'] = "Description for bot with 'newbot' user agent, which will be added to config.robot_regex for exclusion"

  # IP config
  config.ignore_ip_addresses << '' # or regex: /81\.19\.48\.[0-9]+/

  # or provide your own filter functionality, the default is proc{ |request| is_robot? || is_ignored_ip_address? }
  config.ignore_filter = proc{ |request| CustomExcludeLogic.excludes?(request) }

Experiment configuration

Instead of providing the experiment options inline, you can store them in a hash. This hash can control your experiment's alternatives, weights, algorithm and if the experiment resets once finished:

Split.configure do |config|
  config.experiments = {
    "my_first_experiment" => {
      :alternatives => ["a", "b"],
      :resettable => false
    "my_second_experiment" => {
      :algorithm => 'Split::Algorithms::Whiplash',
      :alternatives => [
        { :name => "a", :percent => 67 },
        { :name => "b", :percent => 33 }

You can also store your experiments in a YAML file:

Split.configure do |config|
  config.experiments = YAML.load_file "config/experiments.yml"

You can then define the YAML file like:

    - a
    - b
    - name: a
      percent: 67
    - name: b
      percent: 33
  resettable: false

This simplifies the calls from your code:





You might wish to track generic metrics, such as conversions, and use those to complete multiple different experiments without adding more to your code. You can use the configuration hash to do this, thanks to the :metric option.

Split.configure do |config|
  config.experiments = {
    "my_first_experiment" => {
      :alternatives => ["a", "b"],
      :metric => :my_metric,

Your code may then track a completion using the metric instead of the experiment name:


You can also create a new metric by instantiating and saving a new Metric object.



You might wish to allow an experiment to have multiple, distinguishable goals. The API to define goals for an experiment is this:

ab_test({"link_color" => ["purchase", "refund"]}, "red", "blue")

or you can you can define them in a configuration file:

Split.configure do |config|
  config.experiments = {
    "link_color" => {
      :alternatives => ["red", "blue"],
      :goals => ["purchase", "refund"]

To complete a goal conversion, you do it like:

finished("link_color" => "purchase")

DB failover solution

Due to the fact that Redis has no automatic failover mechanism, it's possible to switch on the db_failover config option, so that ab_test and finished will not crash in case of a db failure. ab_test always delivers alternative A (the first one) in that case.

It's also possible to set a db_failover_on_db_error callback (proc) for example to log these errors via Rails.logger.


You may want to change the Redis host and port Split connects to, or set various other options at startup.

Split has a redis setter which can be given a string or a Redis object. This means if you're already using Redis in your app, Split can re-use the existing connection.

String: Split.redis = 'localhost:6379'

Redis: Split.redis = $redis

For our rails app we have a config/initializers/split.rb file where we load config/split.yml by hand and set the Redis information appropriately.

Here's our config/split.yml:

development: localhost:6379
test: localhost:6379
staging: redis1.example.com:6379
fi: localhost:6379
production: redis1.example.com:6379

And our initializer:

rails_root = ENV['RAILS_ROOT'] || File.dirname(__FILE__) + '/../..'
rails_env = ENV['RAILS_ENV'] || 'development'

split_config = YAML.load_file(rails_root + '/config/split.yml')
Split.redis = split_config[rails_env]


If you're running multiple, separate instances of Split you may want to namespace the keyspaces so they do not overlap. This is not unlike the approach taken by many memcached clients.

This feature is provided by the redis-namespace library, which Split uses by default to separate the keys it manages from other keys in your Redis server.

Simply use the Split.redis.namespace accessor:

Split.redis.namespace = "split:blog"

We recommend sticking this in your initializer somewhere after Redis is configured.

Outside of a Web Session

Split provides the Helper module to facilitate running experiments inside web sessions.

Alternatively, you can access the underlying Metric, Trial, Experiment and Alternative objects to conduct experiments that are not tied to a web session.

# create a new experiment
experiment = Split::Experiment.find_or_create('color', 'red', 'blue')
# create a new trial
trial = Split::Trial.new(:experiment => experiment)
# run trial
# get the result, returns either red or blue

# if the goal has been achieved, increment the successful completions for this alternative.
if goal_acheived?


By default, Split ships with Split::Algorithms::WeightedSample that randomly selects from possible alternatives for a traditional a/b test. It is possible to specify static weights to favor certain alternatives.

Split::Algorithms::Whiplash is an implementation of a multi-armed bandit algorithm. This algorithm will automatically weight the alternatives based on their relative performance, choosing the better-performing ones more often as trials are completed.

Users may also write their own algorithms. The default algorithm may be specified globally in the configuration file, or on a per experiment basis using the experiments hash of the configuration file.

To change the algorithm globally for all experiments, use the following in your initializer:

Split.configure do |config|
  config.algorithm = Split::Algorithms::Whiplash



Ryan bates has produced an excellent 10 minute screencast about split on the Railscasts site: A/B Testing with Split



Special thanks to the following people for submitting patches:

  • Lloyd Pick
  • Jeffery Chupp
  • Andrew Appleton
  • Phil Nash
  • Dave Goodchild
  • Ian Young
  • Nathan Woodhull
  • Ville Lautanala
  • Liu Jin
  • Peter Schröder


Source hosted at GitHub. Report Issues/Feature requests on GitHub Issues. Discussion at Google Groups

Tests can be ran with rake spec

Note on Patches/Pull Requests

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Add documentation if necessary.
  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send a pull request. Bonus points for topic branches.


Copyright (c) 2013 Andrew Nesbitt. See LICENSE for details.