📈 The Rack Based AB testing framework
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Latest commit 9f44c77 Jul 17, 2016 @andrew andrew v2.0.0



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.

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Split currently requires Ruby 1.9.2 or higher. If your project requires compatibility with Ruby 1.8.x and Rails 2.3, please use v0.8.0.

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.


gem install split


Adding gem 'split' to your Gemfile will autoload it when rails starts up, as long as you've configured redis it will 'just work'.


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:

require 'split'

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.

ab_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| %>
  <%= image_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! <% ab_finished(:signup_page_redesign) %>

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

Statistical Validity

Split has two options for you to use to determine which alternative is the best.

The first option (default on the dashboard) uses a z test (n>30) for the difference between your control and alternative conversion rates to calculate statistical significance. This test 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. Split will only tell you if your experiment is 90%, 95%, or 99% significant, and this test only works if you have more than 30 participants and 5 conversions for each branch.

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.

The second option uses simulations from a beta distribution to determine the probability that the given alternative is the winner compared to all other alternatives. You can view these probabilities by clicking on the drop-down menu labeled "Confidence." This option should be used when the experiment has more than just 1 control and 1 alternative. It can also be used for a simple, 2-alternative A/B test.


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' => 18}, {'New' => 2})

ab_test(:homepage_design, 'Old', {'New' => 1.0/9})

ab_test(:homepage_design, {'Old' => 9}, 'New')

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.

In the event you want to disable all tests without having to know the individual experiment names, add a SPLIT_DISABLE query parameter.


It is not required to send SPLIT_DISABLE=false to activate Split.

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. If a test is deleted from the Split dashboard, then it can only be started after pressing the Start button whenever being re-initialized.

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 ab_finished method:

ab_finished(:experiment_name, reset: false)

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

Any old unfinished experiment key will be deleted from the user's data storage if the experiment had been removed or is over and a winner had been chosen. This allows a user to enroll into any new experiment in cases when the allow_multiple_experiments config option is set to false.

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 set the allow_multiple_experiments config option to true like so:

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

This will allow the user to participate in any number of experiments and belong to any alternative in each experiment. This has the possible downside of a variation in one experiment influencing the outcome of another.

To address this, setting the allow_multiple_experiments config option to 'control' like so:

Split.configure do |config|
  config.allow_multiple_experiments = 'control'

For this to work, each and every experiment you define must have an alternative named 'control'. This will allow the user to participate in multiple experiments as long as the user belongs to the alternative 'control' in each experiment. As soon as the user belongs to an alternative named something other than 'control' the user may not participate in any more experiments. Calling ab_test() will always return the first alternative without adding the user to that experiment.

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

By default, cookies will expire in 1 year. To change that, set the persistence_cookie_length in the configuration (unit of time in seconds).

Split.configure do |config|
  config.persistence = :cookie
  config.persistence_cookie_length = 2592000 # 30 days

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: -> (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")
  • expire_seconds: sets TTL for user key. (if a user is in multiple experiments most recent update will reset TTL for all their assignments)

Dual Adapter

The Dual Adapter allows the use of different persistence adapters for logged-in and logged-out users. A common use case is to use Redis for logged-in users and Cookies for logged-out users.

cookie_adapter = Split::Persistence::CookieAdapter
redis_adapter = Split::Persistence::RedisAdapter.with_config(
    lookup_by: -> (context) { context.send(:current_user).try(:id) },
    expire_seconds: 2592000)

Split.configure do |config|
  config.persistence = Split::Persistence::DualAdapter.with_config(
      logged_in: -> (context) { !context.send(:current_user).try(:id).nil? },
      logged_in_adapter: redis_adapter,
      logged_out_adapter: cookie_adapter)
  config.persistence_cookie_length = 2592000 # 30 days

Custom Adapter

Your custom adapter needs to implement the same API as existing adapters. See Split::Persistence::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  = :log_trial # run on every trial
  config.on_trial_choose   = :log_trial_choose # run on trials with new users only
  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(trial)
  logger.info "experiment=%s alternative=%s user=%s" %
    [ trial.experiment.name, trial.alternative, current_user.id ]

def log_trial_choose(trial)
  logger.info "[new user] 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|
  # after experiment reset or deleted
  config.on_experiment_reset  = -> (example) { # Do something on reset }
  config.on_experiment_delete = -> (experiment) { # Do something else on delete }
  # before experiment reset or deleted
  config.on_before_experiment_reset  = -> (example) { # Do something on reset }
  config.on_before_experiment_delete = -> (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, via: [:get, :post, :delete], constraints: -> (request) do
  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 = -> (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
  config.redis_url = "custom.redis.url:6380"

Split looks for the Redis host in the environment variable REDIS_URL then defaults to localhost:6379 if not specified by configure block.


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 = -> (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 can also add meta data for each experiment, very useful when you need more than an alternative name to change behaviour:

Split.configure do |config|
  config.experiments = {
    my_first_experiment: {
      alternatives: ["a", "b"],
      metadata: {
        "a" => {"text" => "Have a fantastic day"},
        "b" => {"text" => "Don't get hit by a bus"}
    - a
    - b
      text: "Have a fantastic day"
      text: "Don't get hit by a bus"

This allows for some advanced experiment configuration using methods like:

trial.alternative.name # => "a"

trial.metadata['text'] # => "Have a fantastic day"

or in views:

<% ab_test("my_first_experiment") do |alternative, meta| %>
  <%= alternative %>
  <small><%= meta['text'] %></small>
<% end %>


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:

ab_finished(link_color: "purchase")

Note that if you pass additional options, that should be a separate hash:

ab_finished({ link_color: "purchase" }, reset: false)

NOTE: This does not mean that a single experiment can complete more than one goal.

Once you finish one of the goals, the test is considered to be completed, and finishing the other goal will no longer register. (Assuming the test runs with reset: false.)

Good Example: Test if listing Plan A first result in more conversions to Plan A (goal: "plana_conversion") or Plan B (goal: "planb_conversion").

Bad Example: Test if button color increases conversion rate through multiple steps of a funnel. THIS WILL NOT WORK.

Bad Example: Test both how button color affects signup and how it affects login, at the same time. THIS WILL NOT WORK.

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 ab_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:

split_config = YAML.load_file(Rails.root.join('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::ExperimentCatalog.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_achieved?


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



Over 70 different people have contributed to the project, you can see them all here: https://github.com/splitrb/split/graphs/contributors


Run the tests like this:

# Start a Redis server in another tab.

rake spec

Source hosted at GitHub.

Report Issues/Feature requests on GitHub Issues.

Discussion at Google Groups.

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) 2016 Andrew Nesbitt. See LICENSE for details.