The incorporation of state and birth year fixed effects accounts for any unobserved state-specific covariates that do not change over time, and any birth-year-specific covariates that are constant across state. While this controls for many potential confounders, it falls short of a controlled-experiment, as does any observational study. There are additional limitations that relate to the accuracy of the effect size estimate, including the lack of exact birth date for decedents, and the lack of information on state of residence during the ages 18 to 21. As discussed at length the Supplemental Material, we expect this to lead to an underestimate of the effect size for the association between MLDA and death by suicide or homicide.
