Figure A.3 shows the ratio of cocaine seizures to purchases in four cities. Until 1997, the ratio was larger in Miami than in the other cities. The ratio in Chicago increased over the period and exceeded that in Miami in 1998. By contrast, the ratio in Boston decreased over time.
Table A.1 provides further evidence of the variation over time and among cities of the characteristics of the STRIDE data. This table shows year-to-year variations in the median quantities of cocaine base purchased in each of four cities during 1990–1998. The variations in the medians are large within cities over time and among cities. For example, the ratio of the highest to lowest median purchase is 13 in Boston and 20 in New York. In 1996, the median quantity purchased in Detroit was 20 times larger than the median quantity purchased in Boston.
STRIDE does not contain information that reveals the reasons for city-to-city and year-to-year variations in the characteristics of the data. It is likely, however, that much of the variation is due to differences in purchase strategies and law enforcement opportunities among cities and years. Miami, for example, is an important port of entry for cocaine, and this accounts for the large ratio of seizures to purchases there. Chicago has become increasingly important as a transshipment point, and this is likely to account for the increase in the ratio of seizures to purchases