Checking Random Assignment in A/B Testing
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Question: In an A/B test, how can you check if assignment to the various buckets was truly random?
To check for random assignment, compare baseline characteristics between groups using descriptive statistics or hypothesis tests (e.g., t-tests for means, chi-square tests for categorical variables).
All covariates (e.g., user age, geography, device type) should be similarly distributed between the groups. If significant differences are found, it could indicate flawed randomization. In such cases, reassigning users or using stratified sampling may be needed.
Look for the main variables and see if there differences in the distributions of the buckets.
Run a linear regression where the dependent variable is a binary variable for each bucket excluding one and the dependent variable is the main kpi you want to measure, if one of those coefficients is significant, you made a mistake.