percentile_ratio is percentile / 100,
e.g. the 90-th percentile corresponds to a percentile_ratio of 0.9.
ddof stands for delta degress of freedom, and the sum of squares will be
divided by count - ddof, where count is the number of elements
for population standard deviation, set ddof to 0
for sample standard deviation, set ddof to 1
ddof stands for delta degress of freedom, and the sum of squares will be
divided by count - ddof, where count is the number of elements
for population variance, set ddof to 0
for sample variance, set ddof to 1