Imagine when you are shopping on Amazon, a list of 50 items is displayed after a search. You scroll down, click an item, continue scrolling, and click on a few more. How does an analyst know if an item has been displayed on the screen to calculate the click rate (clicked/displayed)? How do they know if you saw only 15, or 20, or all 50 items? Is there a scientific way to estimate the furthest point you scrolled base on your clicks, and therefore how many items were actually displayed? It turns out this is “The German Tank Problem”.