Social surveys involve interaction between researcher and the subjects of research, with all the problems this creates for the validity of the measuring instruments being used. Why not, as an alternative, use existing sources of social data which have already been collected for other purposes? For, as Paul Lazarsfeld noted, "man is a data producing animal. Wherever he goes, he leaves certain kinds of data — court records, tax records, school records, birth and death records, and the like. This leads to the possibility of using existing institutional data as indicators of complex social trends and relationships."