What role did the prohibition gangsters play in the 1920s

It limited their activities to prostitution, extortion, gambling, and theft. prohibition provide gangsters with the opportunity to extend their activities in the illegal, and highly lucrative crimes of bootlegging

The answer is "field research".

Field research alludes to gathering essential information from an indigenous habitat without completing a lab test or a review. It is an exploration technique suited to an interpretive system instead of to the logical strategy. To conduct field research, the humanist must will advance into new conditions and watch, take part, or experience those universes. In field work, the sociologists, instead of the subjects, are the ones out of their component.  

The scientist connects with or watches a man or individuals and assembles information along the way.