I think that the original synthetic control paper is this paper by Abadie and colleagues[1]. Figure 1 and 2 give you the intuition behind the need of using the

I think that the original synthetic control paper is this paper by Abadie and colleagues[1]. Figure 1 and 2 give you the intuition behind the need of using the a synthetic control rather than compare California to the rest of the US. You can notice figure 1 that California’s sales of tobacco products was decreasing before the prop 99 and that this trend is different than the rest of the US. This violates the required parallel trends assumption that is required to have an unbiased estimate in a DD model. Abadie et al. reweight the results from the other states to create a synthetic comparison group. This gives them a synthetic California sample that is similar to pre-prop 99 California. You can see that in figure 2. They claim that any deviation from this synthetic group is due to the effects of prop 99 on tobacco sales (Do you believe them? This paper’s conclusion rests on your answer to this question.). Bonus: Figure 3 shows the normalized results. I always appreciate Econ papers that put their results in a graph. Tables are hard to read sometimes. [1]: https://economics.mit.edu/files/11859 https://economics.mit.edu/files/11859