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How to run mutually exclusive experiments in Eppo

Research from Microsoft has shown that in practice interaction effects are vanishingly rare. Given this, we recommend only using mutual exclusion when the overlap of two new treatments critically degrades the user experience. For instance, a design team might want to run a homepage redesign test while a marketing team might want to experiment on the headline messaging on the same page. If the new layout removes the messaging altogether or makes it unreadable, these experiments should clearly not overlap.

If you do find yourself running an experiment where there would be a true interaction effect, we recommend setting up nested feature flags with targeting rules. Using the same homepage example where each experiment has a control and one treatment, you would create 3 feature flags total:

  • one flag called exclusion_group will randomize the users into being in either the copy_test or layout_out test
  • one flag called copy_test will randomize users into the variations set up to test the new messaging against the control
  • one flag called layout_test will randomize users into the variations setup to test the new layout against control

The feature flag naming convention could look like this:

Feature Flag nameVariation NameVariation Name
Feature Flag 1mutual_exclusion_groupcopy_testlayout_test
Feature Flag 2copy_testcopy_controlcopy_treatment
Feature Flag 3layout_testlayout_controllayout_treatment

Follow the quickstart here for additional guidance on how to set up an Eppo Feature Flag in the app.

Eppo App Setup

For your first flag, your setup should look like this:

Screenshot 2023-11-06 at 3.51.28 PM.png

Your copy_test flag should look like this:

Screenshot 2023-11-06 at 3.59.49 PM.png

Where the targeting traits are set up like this:

Screenshot 2023-11-06 at 3.57.50 PM.png

Repeat the feature flag setup and allocation for your layout_test flag:

Screenshot 2023-11-06 at 4.02.24 PM.png

Screenshot 2023-11-06 at 4.04.00 PM.png

Lastly, make sure that you set up the copy_test and layout_test Feature Flags as Experiments in Eppo with the metrics that make sense for each respective experiment.

Eppo SDK setup

The SDK setup would look like this in your application:

import * as EppoSdk from "@eppo/node-server-sdk";

const eppoClient = EppoSdk.getInstance();

// randomize assignment for which exclusion group they will be in
const exclusion_group = eppoClient.getStringAssignment(

// user profile
let user = {
userId: 'unique userid',
exclusion_group: exclusion_group // make sure this attribute matches with the targeting traits you define in Eppo

// randomize assignment on what variation the user will receive for the Copy Test
const copy_variation = eppoClient.getStringAssignment(
{"exclusion_group": exclusion_group}
// randomize assignment on what variation the user will receive for the Layout Test
const layout_variation = eppoClient.getStringAssignment(
{"exclusion_group": exclusion_group}