Consider a brick and mortar shop doing an A/B test on which poster brings more people into the shop: one of an ice cream, or one of a roaring fireplace. It doe

Consider a brick and mortar shop doing an A/B test on which poster brings more people into the shop: one of an ice cream, or one of a roaring fireplace. It doesn't matter how randomly you divide your A and B groups, if you run your test in summer, you'll get different results to running it in winter.