External validity

External validity is a big word that means how well we can use results from experiments to understand the world outside of the experiment. Imagine you wanted to know if a new medicine helped people get better. You might give the medicine to some people and not give it to others (the group that doesn't get it is called the "control group"). If more people in the group that got the medicine got better than in the control group, that's good news! But, to really know if the medicine is helpful for everyone, we need to make sure the results would be the same if we gave the medicine to different people in different places. That's what external validity is all about - making sure we can trust results from experiments even when we're not doing them in exactly the same way in the exact same place every time. So, external validity is just a way to make sure we can use what we learn in experiments to help people in real life.