  Confidence interval of mean is often used when quoting statistics. The same
rigor is often missing when quoting percentiles and tolerance or percentile
intervals. This article derives the expression for confidence in percentiles of
a sample population. Confidence intervals of median is compared to those of
mean for a few sample distributions. The concept of assurance from reliability
engineering is then extended to percentiles. The assurance level of sorted
samples simply matches the confidence and percentile levels. Numerical method
to compute assurance using Brent's optimization method is provided as an
open-source python package.
