(1) Background: Let the continuous parameter X be a proxy variable for the outcome of an intervention R. Quasi-experimental studies are designed to evaluate the effect of R over X when forming a randomized control group (without the intervention) is impractical or/and unethical. The most popular quasi-experimental design. the difference-in-differences (DID) method. https://www.mariameetsanna.com/product-category/ultra-slim-flat-iron/