Raudenbush, S.W. and Bryk, A.S. (2002). Hierarchical Linear Models (Second Edition). Thousand Oaks: Sage Publications.
Rosenbaum, Paul R.; Rubin, Donald B. (1983). "The central role of the propensity score in observational studies for causal effects".Biometrika 70 (1): 41–55.
Shadish, W. R., Cook, T. D., & Campbell, D. T. (2002). Experimental and quasi-experimental designs for generalized causal inference. Boston, MA: Houghton Mifflin
<http://sitemaker.umich.edu/group-based/optimal_design_software>
Software:
Raudenbush, S. W., et al. (2011). Optimal Design Software for Multi-level and Longitudinal Research (Version 3.01) [Software]. Available from www.wtgrantfoundation.org.
Documentation:
Spybrook, J., et al. (2011). Optimal Design for Longitudinal and Multilevel Research: Documentation for the Optimal Design Software Version 3.0. Available from www.wtgrantfoundation.org.
Schochet Z., Peter (2009). Do typical RCTs of Education Interventions Have Sufficient Statistical Power for Linking Impacts on Teacher Practice and Student Achievement Outcomes. National Center for Education Evaluation and Regional Assistance. http://ies.ed.gov/ncee/pdf/20094065.pdf