“Ideally, questions about comparative effectiveness or safety would be answered using an appropriately designed and conducted randomized experiment. When we cannot conduct a randomized experiment, we analyze observational data. Causal inference from large observational databases (big data) can be viewed as an attempt to emulate a randomized experiment—the target experiment or target trial—that would answer the question of interest. “

Miguel A. Hernán and James M. Robins, Am J Epidemiol. 2016 Apr 15; 183(8): 758–764.

Read the full paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4832051/