My fortune cookie today said “your Sunday will be filled with laughter and joy”. I appreciate the sentiment, but I am not going to fall again for the tricks of probability. I know it’s purely random that this fortune note landed in my lap. I could have ordered some other cuisine and this fortune cookie would have been in the belly of some other human. If I assume that there’s a 50% chance this human believes in fortune cookies, 10% chance of this fortune actually proving to be true, and that these two events are independent, then laws of probability say there’s a 5% chance that this fortune cookie would have found its way to someone who actually believes in it and waits until Sunday to see it realize its potential. This is still 5% more than my belief in fortune cookies, which is zero.
Imagine if person B’s blood pressure reading depends on whether person A receives the blood pressure medicine in a randomized controlled trial. This will be violating Stable Unit Treatment Value Assumption (SUTVA) SUTVA states that the treatment received by an individual should not influence the outcome we see for another individual during the experiment. I know the initial example sounded absurd, so let me try again. Consider LinkedIn A/B testing a new ‘dislike’ reaction for its users, and the gods of fate chose you to be part of the initial treatment group that received this update. Excited after seeing this new update, you use this dislike reaction on my post and send a screenshot to a few of your connections to do the same, who are coincidentally in the control group that did not receive the update. Your connections log in and engage with my posts to use this dislike reaction, but later get disappointed as this new update is not yet available to them. The offices of LinkedIn are tr...

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