A paper by Jackson and Borgatta criticizes the indiscriminate use of factor analysis in the study of ecological differentiation because the derived factors are often assumed to be real and persistent over time. This is especially true for ecological studies of delinquency, where the underlying community dimensions associated with delinquency rates are typically interpreted without regard for the historical setting and the urban dynamics giving rise to such community-level patterns.In this research, the delinquency rates of Chicago local communities are examined for a 30-year period. Although a basic pattern emerges for 1940, 1960, and 1970, the structure of the 1950 data is very different. The implications of this divergence in light of Chicago's recent history are examined.