Observational data is derived from observational studies, which are conducted when specific variables are observed and a link is sought.
Experimental data is derived through experiments in which specific factors are controlled and held constant in order to discover if there is any causation.
Splitting a group into two groups is an example of experimental design. The control group goes about their daily activities as usual. For 30 days, the test group is instructed to drink a glass of wine every night. Then studies on how wine affects sleep can be conducted.