There are many different types of study designs, especially in the health-care field. To investigate various ways to setup a study, as well as the relative strength of each method, you should go to http://www.vox.com/2015/1/5/7482871/types-of-study-design.
Read the article and take notes. Make sure you understand the difference between observational and experimental studies in general and the differences between the types of studies described within each category of observational or experimental.
You will have a quiz on this material after spring break.