On "average causal effects"

Schafer and Kang compared the following methods of estimating average causal effects:

1. Simple difference in means;
2. Regression and ANCOVA;
3. Regression estimation;
4. Matching;
5. Inverse-propensity weighting;
6. Subclassification;
   
7. Weighted residual bias corrections;
8. Weighted regression estimation;
9. Regression estimation with propensity-related covariates.

I like No. 9 and plan to begin from there.