One Change at a Time: Designing Fair Tests

Hi, I'm Atlas, and today we are going to learn the most important rule in engineering testing: change only ONE thing at a time. Every experiment has three kinds of variables. The independent variable is the one thing you decide to change — for example, the material your parachute canopy is made from. The dependent variable is what you measure to see the effect — for example, how many seconds the parachute takes to fall. Dependent means it depends on your change. Everything else — the drop height, the weight attached, the way you release it — stays exactly the same. These are your controlled variables, or controls. Here is why this matters. Imagine testing two parachute designs and also dropping one from twice as high. If one lands faster, was it the parachute design or the lower drop height? You cannot tell, because two things changed at once. A fair test removes that confusion by changing only the independent variable while controls hold everything else steady. Real engineers also run repeated trials — they test each design three or more times and record all the numbers. One trial can fool you. A gust of air, a crooked release, or a measurement slip can give a strange result. More trials average out those one-time flukes and reveal a clearer, more trustworthy pattern. If you ever feel stuck, ask these three questions: What is my ONE change (independent variable)? What am I measuring (dependent variable)? What am I keeping the same (controls)? Answer those three, and you have a fair test.