This technical report addresses the complexities of calculating dropout rates and presents results from one method of calculation that represented Chicago Public Schools (CPS) dropout rates over several years. The goal was to develop an indicator that would provide accurate and comparable measurement over a sufficient length of time to reveal the extent to which CPS dropout rates were declining or rising. Although the methods used to calculate this statistic were based on Hess' procedure (developed in the 1980s), there were several key alterations. The major difference was that in this study, students were grouped into cohorts according to when they reached their 13th birthday, rather than starting the cohort when students entered high school for the first time. Students were followed from ages 13-19 years. The report presents results of the calculations and describes how and why researchers modified Hess' methods, documenting decisions they made on technical questions faced while producing the indicator. Overall, the estimates showed that over 40 percent of CPS students dropped out by age 19 years. The rates decreased by 2.5 percent between the 1991 and 1994 cohorts. Using different criteria to calculate the dropout rates resulted in substantially different numbers. (SM)