Introduction to Difference-in-differences Design

抄録

Because it is difficult to conduct randomized controlled trials, observational studies are often used when evaluating the effects of health care policies. However, observational studies are subject to bias, such as a failure to eliminate the effects of trends in the outcome over time and permanent differences between treatment and control groups. The difference-in-differences design removes these biases by observing outcomes for the two groups at two time points. This article introduces the methods and assumptions for the difference-in-differences design and provides some examples of studies that have used this design.