There are two primary means for collecting data. The essential difference consists of who assigns the treatments. In an experimental study, the researcher has control over the allocation, and in an observational study, the researcher has no control. When there is no control over the allocation, the researcher must decide if the allocation has divided the observations in other ways than those of interest.
Causal inference is the gold standard .
Establishing association is nice, but not nearly as satisfying as establishing cause-effect.