The use of aggregate data in social science research is a controversial topic. It has the advantage of not requiring time-consuming and costly original field research, relying instead on previously accumulated information from censuses, voter registrations and other sources. Yet it has been shown that the results of studies using aggregate data can be tenuous, at least on an individual level. Many of the contributions to this collection of essays first appeared in 1979 issues of Sociological Methods and Research. They focus on the nature of measurement for aggregated data and many of its possibilities and hazards.