Formulas > Descriptive Statistics > First Quartile

The first quartile (Q1) separates the bottom 25% of the sorted values in a data set from the top 75%.

For a data set , in which the values are sorted in increasing order,

Q1 = xi + a(xi+1 - xi)

where
i = floor((n+1)/4)
a = (n+1)/4 - i.