References
- Rubin , D. B. 1974 . “Estimating Causal Effects of Treatments in Randomized and Non-Randomized Studies” . Journal of Educational Psychology , 66 : 688 – 701 .
- Wilcoxon , F. 1949 . Some Rapid Approximate Statistical Procedures , Stamford , CT : Stamford Research Laboratories, American Cyanamid Co. .