Regarding the article "The impact of health insurance and health", by Helen Levy and David Meltzer, the authors summarize the literature on the impact of health insurance on health, and in doing so, they divide studies into three groups: observational, natural, and randomized. How can someone briefly describe and discuss the benefits/limitations of observational studies and natural experiments for this question. How can someone clearly state how these two are different from one another. Explain in one paragraph.