What is Stratified Sampling
Stratified sampling involves dividing the entire population into homogeneous groups called strata (plural for stratum). Random samples are then selected from each stratum. e.g. If you had XS and XL customers and randomized them into two groups - Control and Test, you'd want both Control and Test to be balanced across XS and XL customers.
With large numbers, good randomization typically solves this. However in B2B scenarios and other relatively low volume or high variance scenarios, stratified sampling is useful to ensure this balance. Statsig supports both automated (still in beta) and manual stratified sampling.
Automated Stratified Sampling
This is in development. Reach out in Slack if automated stratified sampling is of interest to you.
Manual assignment for Stratified Sampling
When setting up an experiment, you can configure overrides (e.g. force user X or Segment A into Control, force user Y or Segment B into Test). This is meant for testing; overridden users are excluded from experimental analysis in Pulse results. If you do want manual assignment for stratified sampling, you should check the Include Overrides in Pulse checkbox. This will include the users you've manually overridden into each variant in all metric lift analyses. You can configure 100% of experiment participants into your test variants manually, or configure some subset of participants into variants manually and randomly assign the rest of your participants.
When you use the Statsig SDK for assignment, it takes care of randomization. When you control assignment of users, you're responsible for making sure users are balanced across experiment groups.