@lis Since you're measuring at a full-patient granularity, you have to set the boundaries of your measurement error at 'zero plus or minus half failures'. The study had 2134 participants, and of them, the measured failure rate was less than half out of every 2134, which is some 0.02343%.You don't get to collapse your margin of error to zero merely because the event was in the past. This is not probability; it's uncertainty. These are different things.