Regression discontinuity design

Regression discontinuity design is like a science experiment. You start by taking two groups of people and making them as similar as you can. Then you do something to just one group, like giving them a special treatment or teaching them a new skill. Then you look at how the two groups changed. Were the group with the special treatment or skill better than the group without it? This experiment helps you find out if the special treatment or skill really worked.