Analyze A/B test results in Amplitude

A/B Testing is a method of conducting controlled, randomized experiments with the goal of improving a website or application metric. With Amplitude's AB Test View (available for Growth and Enterprise plans), you can measure the impact of your experiments by comparing how each experiment group behaves in your application.

For example, you can show two different onboarding flows to different groups of new users, then use the results to determine which one is more effective in driving users to complete the onboarding process. Or you can test different checkout flows to see which is more effective in generating sales.

NOTE: This feature is not to be confused with Amplitude Experiment, a more robust, full-featured experimentation platform, or with Experiment Results.

Before you begin: Instrument your experiments

Before you can do anything, however, you'll need to instrument your experiments. We recommend using user properties to associate a user with a given experiment variation. User properties reflect traits about each individual person using your product. Use them to segment your analysis in Amplitude Analytics.

There are two main ways to update a user property:

1) SDKs & HTTP API: Update user properties on event action.

• How: User properties can be sent with each event via our SDKs or HTTP API.
• Pros: User properties will take effect at the moment the event is sent and exist with the user for all subsequent events, until the property values are explicitly updated.
• Cons: These events will count towards your monthly event volume. Further, these events will count users as active users by default, so you'll need to ensure any A/B  testing-related events are marked as inactive events.

2) Identify API: Update user properties without sending an event.

• How: Amplitude's Identify API allows you to update a user property without sending an event.
• Pros: Can asynchronously update a user property without sending an event, and will not impact your monthly event volume count.
• Cons: The user property will not take effect until the user takes an action. This usually is not an issue for most experiments, but it may have an impact on experiments that are aimed to track whether or not inactive users are returning to your application.
  
  For example, suppose you're trying to get users who have been inactive for more than seven days to return to your app, and you're testing the effectiveness of an email to make that happen. If the Identify API is used to update a user property, it will only be applied to those users who have returned to trigger an event—any event, it doesn't matter which—in your application. If a user remains inactive after receiving the email, the user property will not be applied to this user. As a result, this inactive user will not be included in the experiment group that has received the email because the user property never attached to them. In situations like these, we recommend option #1—updating user properties on an event action (eg. an event called "Email Sent").

Learn more about how user properties are synced in Amplitude.

How many user properties should you send?

Amplitude users tend to take one of two approaches when instrumenting split tests:

• Use one user property per experiment.
  
  All user properties are received as key-value pairs. This approach sets the experiment name as the key and all variations of the experiments as the potential values.
  
  User Property Key: Experiment 1
  User Property Value: variation_a
  
  Pros: Can easily select experiments to segment by from the user segmentation tab.
  Cons: Can result in an overwhelming list of user properties, depending on the number of experiments being run.
  
  
• Use one user property for all experiments.
  
  All user properties are received as key-value pairs. This approach sets the key to Split Tests (or something similar) and stores the values in an array.
  
  User Property: Split Tests
  User Property Value: [experiment_1_value, experiment_2_value]
  
  You can segment on the user property Split Tests by selecting the appropriate value or test group in the chart's segmentation module.
  
  Pros: You will only have one user property related to your split testing (rather than one per experiment), so your user property list will be more manageable in the dashboard.
  Cons: Arrays are limited to 10,000 characters if append or prepend is used. If an array were to exceed the character limit, then the characters past the threshold will not be recorded. 

In addition to instrumenting split test experiments on your own, Amplitude offers a full integration with Optimizely that will automatically update user properties for each experiment. Please read the integration guide for more details.

Viewing results in Amplitude

You can begin reviewing the results of your split tests after user properties have been updated for each experiment group. The AB Test View (available for Growth and Enterprise plans) is a feature within the Funnel Analysis chart that will allow you to perform this analysis.

You can compare the activity between experiment groups by using the segmentation module of the chart control panel. To do this, simply add your experimental groups.