How do you design allocation logic to minimize concurrent A/B test interactions?

WHAT THIS TESTS: This question tests whether you can reason about causal inference at platform scale. Interviewers want to see that you recognize concurrent experiments are not independent by default, that overlapping treatments can compound or cancel each other out, and that the allocation layer is the right place to enforce constraints rather than the analysis layer. They also care that you know concrete mechanisms like mutual exclusion and layered randomization with reservation amounts.

A GOOD ANSWER COVERS: First, define interaction effects precisely: when a user is exposed to multiple simultaneous treatments, the observed outcome is not simply the sum of individual effects, so attribution becomes unreliable. Second, explain mutual exclusion: users are bucketed into at most one experiment within a mutually exclusive group, which eliminates cross-experiment interactions entirely but caps the traffic available to each test and slows overall velocity. Third, describe layered randomization: experiments are organized into orthogonal layers where each layer draws an independent randomization unit for the same user, allowing parallel tests to run on the same population without sharing a single bucket space; mention that reservation amounts let you cap what percentage of a layer each experiment can consume so you do not starve future tests. Fourth, discuss the trade-off between purity and throughput, noting that mutual exclusion is best for high-risk or tightly coupled features while layers are better for independent surface areas.

COMMON WRONG ANSWERS: A major red flag is suggesting random assignment into all concurrent experiments with the intention to fix interactions during analysis; post-hoc regression or interaction terms cannot recover clean causal estimates when the allocation was not designed for it. Another mistake is claiming that simply lowering traffic percentages avoids interactions; partial overlap still biases estimates. Some candidates also propose mutual exclusion everywhere, ignoring that it serializes experiments and destroys velocity for large platforms.

LIKELY FOLLOW-UPS: An interviewer might ask how you would detect unexpected interactions if layers are used, how you would handle session consistency when a user moves between devices, or how reservation amounts should change when an experiment is promoted to a full rollout. They might also ask for a hashing scheme that keeps layer assignments stable and orthogonal, such as using a salt per layer with a deterministic hash of the user identifier.

ONE CONCRETE EXAMPLE: Imagine a homepage with three concurrent tests: a headline change, a button color change, and a pricing banner. If all three overlap randomly, the pricing banner might perform differently when paired with a red button versus a blue button, creating a positive interaction that inflates the headline test results. To prevent this, you could create a single layer for homepage UI tests and assign each experiment a reservation amount of thirty percent. Each user gets a deterministic hash within that layer, mapping them into exactly one variant across all three tests while preserving the ability to run them simultaneously. If the pricing banner is business critical and must not interact with anything, you would move it into its own mutually exclusive group instead.

Source: launchdarkly.com