The Commuity Programs for Clinical Research on AIDS compared zalcitabine (ddC) against the control, didanosine (ddI). The number of events on ddC and ddI, respectively are provided in parentheses under the calendar date of each of the 4 analyses. At each analysis, the estimated ddC to ddI hazard ratio for the rate of progression to symptomatic AIDS or death is represented by asterisks, the square brackets represent nominal 95% confidence intervals, and the orange parentheses represent the O'Brien-Fleming repeated confidence intervals properly adjusting for the group sequential analyses.