Difference-in-differences (DID) methods is used to evaluate the impact of different policies. the simplest of these DID is when there are two groups and two time periods. where one of the groups is exposed to the treatment in the second second period and the control group is not exposed to the treatment in both periods, and both two groups are followed in two time periods. From this short review, I would like to share the findings of Okoro et al. (2014) on the effects of MA health reform on the use of clinical preventive services . The authors compared the change in health care access as a result of the expansion of the health insurance coverage. the authors compared MA with other New England states (ONES). To do so, they used BRFSS, and as well they looked at different preventive services and health access by age group and gender over different time periods both for ONES and MA. The authors found a significant improve in MA in access to care. For clinical preventive services, MA showed to have greater increases in colorectal cancer screening and not to have had a decline in cervical cancer screening as was observed in the ONES. These results are significant but they can’t be generalized to other states, which decides to expand their health insurance coverage. More research is needed to be conducted, which control of health behavior of the population from which states. Reference: Okoro, C. A., Dhingra, S. S., Coates, R. J., Zack, M., & Simoes, E. J. (2014). Effects of Massachusetts health reform on the use of clinical preventive services. Journal of general internal medicine, 29(9), 1287-1295.