Stratified sampling

This topic explains how to use stratified sampling to re-randomize the contexts in your experiment treatments. Re-randomization helps eliminate covariate imbalance and reduce false positives and false negatives in your experiment results.

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Covariate imbalance and re-randomization

Covariate imbalance is a well-known problem in randomized controlled trials, such as experiments, where the control and treatment groups look significantly different from each other, despite the experiment audience being allocated by random chance. One common example of this in business-to-business (B2B) experiments is when the vast majority of users are low spenders, but a handful are very large spenders (sometimes called “whales”). A simple random sample can result in the scenario where all of the whales end up in one variation, creating an imbalance. Covariate imbalance can cause false positives or false negatives, because the differences in treatment audiences can overwhelm or obscure differences due to the effect of the treatment. Experiment re-randomization using stratified sampling is a way to mitigate this problem by using data available prior to the experiment to balance treatment allocation. When you have a pool of known contexts, such as users, and their associated metadata such as size or country, you can use this information to re-randomize your treatments. Re-randomization works by:

• Generating many possible variation pool randomization options
• Generating statistical measures of the quality of balance of each randomization with respect to your given covariate
• Selecting the randomization with the best balance according to that measure

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Example of stratified sampling in use

Imagine you’re a B2B e-commerce business running an experiment on your checkout flow with revenue as the success metric. Suppose you know that within your customer base, about 10% of your customers are “large” and 90% of your customers are “small”, where:

• Large customers have mean revenue of $1000 per day
• Small customers have mean revenue of $10 per day

Randomly assigning users to a variation, unaware of the user size, is called simple random sampling (SRS). An experiment using SRS, with a large enough sample of users, is almost guaranteed to have a distribution of 10% large and 90% small users in each treatment. However, a small sample is more likely to produce covariate imbalance. Imagine you run an A/A test to check the setup of your experiment. Using SRS might result in a randomization with customer size distribution like the following:

• Control: 10% large, 90% small
• Treatment: 5% large, 95% small

Given that this is an A/A test, the treatment is expected to perform roughly as well as the control. So on average, we would expect the following mean daily revenues in each treatment:

• Control: 0.10 * $1000 + 0.90 * $10 = $109 per user
• Treatment: 0.05* $1000 + 0.95 * $10 = $59.5 per user

Although there should be no difference in the two variations, you will see almost a 2x difference between the two treatments, purely due to the composition of the users within each treatment. Re-randomization addresses this by giving the experiment a way to ensure that the balance between large and small users in the control and treatment are the same, or close to the same.

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Stratified sampling in LaunchDarkly experiments

When you create a LaunchDarkly experiment, you can upload a CSV file populated with context keys and covariate values to reduce covariate imbalance. Your CSV should include known context keys that cover the full range of your covariate values. For example, if you have customers that fall into “small” and “large” buckets, you should include many contexts for each value. Your CSV file should have the following structure:

• The first line must include the column names: context_key,covariate_value.
• Subsequent lines should contain the context key and its covariate value, separated by a comma. For example: example-context-key,large.
• The file may define additional columns and column data, but LaunchDarkly ignores this information.