  For a panel model considered by Abadie et al. (2010), the counterfactual
outcomes constructed by Abadie et al., Hsiao et al. (2012), and Doudchenko and
Imbens (2017) may all be confounded by uncontrolled heterogenous trends. Based
on exact-matching on the trend predictors, I propose new methods of estimating
the model-specific treatment effects, which are free from heterogenous trends.
When applied to Abadie et al.'s (2010) model and data, the new estimators
suggest considerably smaller effects of California's tobacco control program.
