$3.(10$ pts $)$ About the DDD model...

a$)(3$ pts $)$ What is a difference$-$in$-$difference$-$in$-$difference $($DDD$)$ model?

b$)(3$ pts$)$ Suppose we obtain a $"$DD of interest" for our primary research question. Then, by introducing an alternative control group and an alternative outcome variable, we get the "placebo DD$."$ What is the DDD estimator as a function of $"$DD of interest" and "placebo DD$"?$

c$)(4$ pts$)$ Why is the DDD model of little research interest?