I was fascinated to observe this allergy test. A strip is stuck on and 10 possible allergens are separately dropped on and poked into the skin. The ++ at the top is the control. On the left side it is histamine to show the way that chemical makes the skin come up in a welt. The control is needed to show that the skin really does react to histamine. A negative control was placed on the right hand side. I didn't find out what it was, maybe antihistamine or perhaps just water. You can't tell because nothing happened to the victim. It is to show that the skin doesn't react to just anything. Now compare the welt at 4 in the top picture with the control. Allergen 4 must be provoking the body to produce histamine and that is an allergic reaction. It is a great example of a controlled experiment. It's not a technique used in Physics. I'm struggling to think of an example. You can't have a control Universe to measure our Universe against!
