In 2024, for JHU's day of giving, our department sent out postcards. The A/B testing in this case was for postcard size. The postcards looked identical (shown in attached image), but one was 4 x 6 inches and the other 8.5 x 5.5 inches. Gifts from people who had received postcards were tracked. At the beginning, it seemed like the smaller postcard influenced giving, but as the campaign progressed, it seemed as though postcard size had no effect. The team learned afterwards that the smaller postcards were marked delivered at much earlier dates, which explained why they seemed to have an outsized effect at the start of the campaign.