You want to test a change that alters comment sorting in a subreddit, but users participate in multiple subreddits and may share posts across them. How do you choose the unit of randomization and analysis to handle interference on the social graph?

Interview question asked to Data Scientists interviewing at Postmates, LiveRamp, BetterUp and other companies. Original question asked: You want to test a change that alters comment sorting in a subreddit, but users participate in multiple subreddits and may share posts across them. How do you choose the unit of randomization and analysis to handle interference on the social graph?.