In what circumstances do scientists use observational research and opinion based research?

Whenever it is not possible for a scientist to do controlled experiments, then they use observational research and opinion based research. Observational research actually requires collecting of data without performing an experiment. These observations often lead to biased and inaccurate results. This is one of the major pitfalls of observational research. Opinion based research often requires a scientist to design an experiment and collect the data. The data collected are mostly arbitrary in nature, but it definitely gives a direction for further progress.